[AMDGPU][mc][tests][NFC] Add coverage/smoke tests for Gfx7 and Gfx8.

Each test contains 20-30K test cases but takes only several (from 4 to 10)
seconds to complete on average machine. The tests cover the majority of
AMDGPU Gfx7/Gfx8 instructions, including many dark corners, and intended
to quickly find out if something is broken.

llvm-svn: 292922