AMDGPU/MC: Simplify AsmParser for VOP3P

Remove check that instruction has src_modifiers because
isRegOrImmWithInputMods essentially includes this check.
isMod is no longer used after the refactor. Also, was unreachable case
inside for loop. clamp and omod operands are handled after the loop.

Differential Revision: https://reviews.llvm.org/D139194