[llvm] [BOLT] Prevent missing ExternalBranch validation (PR #221927)

[via llvm-commits]

https://github.com/Thrrreeee approved this pull request.

LGTM. 
This fixes a crash we encountered in BOLT for RISC-V when a single source function branches to constant islands in multiple target functions. Tracking validation at the branch level provides finer granularity than the previous function-level approach and ensures that no invalid targets are missed.

BTW, could you also add a corresponding RISC-V test case? If you’re not familiar with that area, I can add one in a follow-up.

https://github.com/llvm/llvm-project/pull/221927