[AMDGPU] Coverage tests for instruction sets

Hi all,

I’m going to submit three relatively big (1.6M, 2.7M and 3.9M) lit tests to test/MC/AMDGPU and test/MC/Disassembler/AMDGPU.

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 implemented AMDGPU instructions, including some dark corners, and intended to quickly find out if something is broken.

Right now all tests PASS, of course.

Note that llvm contains similar tests, for example test/MC/X86/intel-syntax-avx512.s which is ~1.6M.

Please let me know if any objections.