[llvm] [RFC][MC] Cache MCRegAliasIterator (PR #93510)

[Pierre van Houtryve via llvm-commits]

Pierre-vh wrote:

> The real fix should be to switch all uses of MCRegAliasIterator to use MCRegUnitIterator instead. No liveness infrastructure should operate in terms of aliases

I'm not too familiar with reg units but as far as I understand them, yes, that'd probably make more sense.

However I'm wondering how realistic it is to expect that itt'll happen anytime soon. I can fix one or two cases myself I think but we have about 80 uses in llvm/lib. 

How do you feel about caching? Do you think it's a good addition nonetheless, or should we avoid it in favor of (hopefully, someday) fixing all the unnecessary RegAliasIterator uses?

https://github.com/llvm/llvm-project/pull/93510