[AMDGPU][AsmParser][NFC] Simplify the EndpgmImm operand definition.

Clears the road to eliminating custom default operand handlers. Also
unifies naming of related entities.

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

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D151687