Summary

This PR improves how the compiler handles reassigned locals, especially across if branches, and reduces redundant stack work when a reassigned value is consumed for the last time.

What changed

• Fixed stack-local reassignment cleanup so the compiler removes the correct stack slot instead of assuming the old value is always on top.
• Kept overwritten stack slots tracked as hidden shadow bindings until cleanup, so later stack-depth calculations stay correct after reassignment.
• Used OpRoll instead of OpPick in the targeted reassignment cases where a stack value is being consumed for the last time, which avoids extra cleanup.
• Made if normalization more reliable when outer variables are reassigned in only one branch.
• Ensured synthetic __if_* values are materialized consistently across branches so merged stack and type state stay correct.
• Cleaned up branch-local hidden shadow bindings so they do not leak into later control flow.