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

[Pierre van Houtryve via llvm-commits]

Pierre-vh wrote:

> Maybe it's fine for a stopgap

It's what I think too, but as the saying goes, there's nothing as permanent as a temporary solution. I'm not comfortable landing this unless everyone is sure it'd beneficial even if we end up removing most uses of RegAlias iterator at some point.

I personally think it is beneficial anyway because:
* If we remove RegAlias iterator as a whole at some point, then this just goes away.
* If we can't remove it and there are still some uses left in the hot path, caching is always a good thing.

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