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

[Sergei Barannikov via llvm-commits]

s-barannikov wrote:

+1 for switching to regunit iterators

> There's some additional infrastructure changes that would make this easier, like moving block live ins and reserved registers to directly use regunits

This is what I'd like to see, too. It is not obvious how to deal with e.g. X86 EAX high part

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