[llvm] [PGO] Add `llvm.loop.estimated_trip_count` metadata (PR #148758)

[Mircea Trofin via llvm-commits]

https://github.com/mtrofin commented:

still reading through, and sorry for the delay on my end, I was on vacation. One high level question, should this maybe also touch the bitcode validator? 

https://github.com/llvm/llvm-project/pull/148758