[AMDGPU] Add gfx8 assembler and disassembler test cases

Summary:
This adds assembler tests for cases that were previously only in the
disassembler tests, and vice versa.

Reviewers: rampitec, arsenm, nhaehnle

Subscribers: kzhuravl, jvesely, wdng, yaxunl, dstuttard, tpr, t-tye, jfb, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D72561