Create helper functions to support plan_rewrites() extraction:
- build_local_block_map(): Build block ID mapping for a function
- sync_spans(): Synchronize instruction spans after rewriting
These pure functions will be used by both the current monolithic
merge_and_rewrite() and the new plan_rewrites() function.
Progress: Step 1/4 (helpers) complete
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
tomoaki
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