[llvm] [X86] Reduce znver3/4 LoopMicroOpBufferSize to practical loop unrolling values (PR #91340)

[via llvm-commits]

ganeshgit wrote:

> isn't it limited by the 6 wide renamer in practice

I am not sure why you mention renamer. It has a role to play in the throughput however it is not strictly limited to renamer's capacity right? Renamer's capability will be on case-case basis but theoretical limit will be 9uops.  Also, renamer can be split across  integer\fp(vector). So, I don't think we should restrict it with renamer's capability. 

@RKSimon I think we should add a tuning flag whether a subtarget is willing to use this LoopMicroOpBufferSize for unrolling decision. I agree that the metric you are proposing is serving the purpose but the term LoopMicroOpBufferSize in itself is misleading and is not representative. 

https://github.com/llvm/llvm-project/pull/91340