[AMDGPU] Refactor SOPC & SOPP .td for extension

We use the Real vs Pseudo instruction abstraction for other
types of instructions to facilitate changes in opcode
between gpu generations.
This patch introduces that abstraction to SOPC and SOPP.

Reviewed By: rampitec

Differential Revision: https://reviews.llvm.org/D89738

Change-Id: I59d53c2c7058b49d05b60350f4062a9b542d3138