[AMDGPU] Add & use `hasNamedOperand`, NFC

In a lot of places, we were just calling `getNamedOperandIdx` to check if the result was != or == to -1.
This is fine in itself, but it's verbose and doesn't make the intention clear, IMHO. I added a `hasNamedOperand` and replaced all cases I could find with regexes and manually.

Reviewed By: arsenm, foad

Differential Revision: https://reviews.llvm.org/D137540