fix: hoist allocas out of conditional branches in split and lastIndexOf

[cs01]

Summary

• Hoists 8 alloca instructions from inside conditional branches to the function entry block in generateSplit (6 allocas) and generateLastIndexOfFrom (2 allocas)
• Prevents undefined behavior where LLVM stack slots may not exist when accessed from other basic blocks
• Same class of bug as fix: hoist alloca out of conditional branch in generateReplaceAll #298 (replaceAll)

Test plan

• npm run verify:quick passes (tests + Stage 1 self-hosting)
• All string split and lastIndexOf tests continue to pass

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