[llvm] [AMDGPU][GlobalIsel] Introduce isRegType to check for legal types, instead of checking bit width. (PR #68189)

[via llvm-commits]

sstipanovic wrote:

> 2. It looks like `isRegisterClassType` will not be super fast, especially since it has to construct all the pointer types on every call.
@jayfoad  `initializer_list` makes things tricky. I can make pointer types static to avoid having them constructed on each call. What do you think?


https://github.com/llvm/llvm-project/pull/68189