SROA: preserve load annotations

[zhouguangyuan0718]

Summary

• copy load annotation metadata when SROA splits or rewrites loads
• preserve standard frontend annotations through optimization without adding Go-specific SROA behavior
• add a focused transformation test

Relationship

This generic change is split from LLVM PR #17 after LLVM PR #18 merged. It is independent of write-barrier lowering and unsafe-point metadata. Go PR goallc/go#36 uses the preserved annotation for frontend nil checks.

Validation

• focused LLVM lit: preserve-load-annotation.ll, 1/1
• opt default integration with cmd/llvmplugin: use standard annotations for nil checks go#36
• git diff --check

[github-actions]

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