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

[Joel E. Denny via llvm-commits]

jdenny-ornl wrote:

I believe I have addressed most issues raised in PR #148758.

For now, this PR shows what the pass pipeline tests (e.g., `llvm/test/Other/new-pm-defaults.ll`) look like after recent changes.  [As @nikic pointed out](https://github.com/llvm/llvm-project/pull/148758#pullrequestreview-3076627201), there are still changes to where DominatorTreeAnalysis and LoopAnalysis run.  LastRunTrackingAnalysis too.

I have not addressed the general issue raised there about whether pgo-estimated-trip-counts is actually worthwhile and whether we should constrain it or disable it by default.  What do people think now after seeing the above changes?

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