[llvm] [PGO] Add llvm.loop.estimated_trip_count metadata (PR #152775)

[Joel E. Denny via llvm-commits]

jdenny-ornl wrote:

> LLVM Buildbot has detected a new failure on builder `cross-project-tests-sie-ubuntu` running on `doug-worker-1a` while building `llvm` at step 6 "test-build-unified-tree-check-cross-project".
> 
> Full details are available at: https://lab.llvm.org/buildbot/#/builders/181/builds/27713

The bot is green now without a revert.  I assume it's just flaky.

https://github.com/llvm/llvm-project/pull/152775