ZJIT: Optimize send with nil block arg to SendDirect

[rwstauner]

When a send has ARGS_BLOCKARG and the block arg is nil (either statically known or profiled as monomorphically nil), we can strip the block arg and optimize the call to SendDirect without a block.

This reduces the param_block category in the complex argument-parameter fallback stats (34.2% of total on lobsters as of the burndown in #833).

Implementation

• Profile the block arg alongside self and regular args for send instructions
• If the block arg type is statically NilClass or profiled as monomorphically nil, strip it and emit SendDirect without a block
• Insert a GuardBitEquals side exit when relying on profiled (not static) nil
• Update the frame state snapshot after stripping to keep SP consistent in codegen

Bug fix

The initial implementation had a stack consistency error (sp: N+1, bp: N) when the guard side-exited for a non-nil block. The frame state snapshot still contained the stripped block arg, causing gen_save_sp to compute the wrong SP. Fixed by creating a new snapshot with the block arg removed.

Related: #833 (lobsters perf burndown)