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

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

> @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.

We shouldn't need a TLI/TTI control for this - either removing the LoopMicroOpBufferSize entry (see znver1/2) or explicitly setting it to 0 has a similar effect. But I'm not certain if we want to do this for znver3/4 or not - I don't have access to hardware to test this.

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