[mypyc] Fix range loop variable off-by-one after loop exit

[VaggelisD]

Fixes mypyc/mypyc#1191

Previously, ForRange.gen_step() updated both the internal index register and the user-visible loop variable after incrementing. This meant the loop variable was set to the incremented value before the condition check could reject it, causing an off-by-one overshoot on loop exit.

• Before:

  ┌─────────────────────────────────────┐                                                                                                                                                      
  │ L1: condition                       │                                                                                                                                                      
  │   if index_reg < end → L2 else L4   │                                                                                                                                                       
  └──────────────┬──────────────────┬───┘                                                                                                                                                      
                 │ true             │ false                                                                                                                                                    
                 ▼                  │                                                                                                                                                          
  ┌─────────────────────────────────┐   │                                                                                                                                                      
  │ L2: body                        │   │                         
  │   ... use index_target ...      │   │                                                                                                                                                      
  └──────────────┬──────────────────┘   │                                                                                                                                                      
                 ▼                      │
  ┌─────────────────────────────────┐   │                                                                                                                                                      
  │ L3: step                        │   │                         
  │   index_reg = index_reg + step  │   │                                                                                                                                                      
  │   index_target = index_reg  ◄── BUG │
  │   goto L1                       │   │                                                                                                                                                      
  └─────────────────────────────────┘   │                         
                                        ▼                                                                                                                                                      
                                ┌───────────────┐                 
                                │ L4: exit      │                                                                                                                                              
                                │ index_target  │
                                │ = overshot!   │                                                                                                                                              
                                └───────────────┘                                                                                                                                              

• After:

  ┌─────────────────────────────────────┐
  │ L1: condition                       │
  │   if index_reg < end → L2 else L4   │
  └──────────────┬──────────────────┬───┘                                                                                                                                                      
                 │ true             │ false
                 ▼                  │                                                                                                                                                          
  ┌─────────────────────────────────┐   │                         
  │ L2: body                        │   │                                                                                                                                                      
  │   index_target = index_reg  ◄── FIX │
  │   ... use index_target ...      │   │                                                                                                                                                      
  └──────────────┬──────────────────┘   │                         
                 ▼                      │                                                                                                                                                      
  ┌─────────────────────────────────┐   │
  │ L3: step                        │   │                                                                                                                                                      
  │   index_reg = index_reg + step  │   │                         
  │   goto L1                       │   │
  └─────────────────────────────────┘   │                                                                                                                                                      
                                        ▼
                                ┌───────────────┐                                                                                                                                              
                                │ L4: exit      │                 
                                │ index_target  │
                                │ = correct ✓   │
                                └───────────────┘ 

[hauntsaninja]

Thanks for the fix!