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

[Jay Foad via llvm-commits]

jayfoad wrote:

There is also some relevant discussion on https://reviews.llvm.org/D148096 which introduced isIllegalRegisterType.

Do we really need both isIllegalRegisterType and isRegType?

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