[AMDGPU] Deduplicate VOP tablegen asm & ins

VOP3 and VOP DPP subroutines to generate input
operands and asm strings were essentially copy
pasted several times. They are deduplicated to
reduce the maintenance burden and allow faster
development.

Reviewed By: dp

Differential Revision: https://reviews.llvm.org/D94102

Change-Id: I76225eed3c33239d9573351e0c8a0abfad0146ea