[llvm] [LoopUnroll] Clamp PartialThreshold for large LoopMicroOpBufferSize (PR #67657)

[Simon Pilgrim via llvm-commits]

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

I think this is a good generic limit to put in place so the numbers are compatible with what the loop transforms expect. I'm still hoping we can come up with some solution on #91340 for znver3/4 specifically but that's getting hindered by lack of test hardware.

https://github.com/llvm/llvm-project/pull/67657