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

Reason: broke ASAN bots with a global buffer overflow.
http://lab.llvm.org:8011/builders/sanitizer-x86_64-linux-fast/builds/2291

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: 292974