[llvm] [MachinePipeliner] Fix fragile tests (PR #123169)

[Ryotaro Kasuga via llvm-commits]

https://github.com/kasuga-fj commented:

I modified #121907 a bit and now performance regressions are resolved for the cases here. But I'm still concerned about any other regressions in Hexagon benchmarks.

https://github.com/llvm/llvm-project/pull/123169