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

[via llvm-commits]

sstipanovic wrote:

> There is also some relevant discussion on https://reviews.llvm.org/D148096 which introduced isIllegalRegisterType.
> 
> Do we really need both isIllegalRegisterType and isRegType?

I think once we replace all usage of isRegisterType we could do without isIllegalRegisterType.

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