[AMDGPU][AsmParser] Refine parsing of some 32-bit instruction operands.

Eliminates the need for the custom code in parseCustomOperand().

The remaining uses of NamedOperandU32 are to be addressed separately.

Part of <https://github.com/llvm/llvm-project/issues/62629>.

Reviewed By: dp

Differential Revision: https://reviews.llvm.org/D150204