[NFC][AMDGPU] Refactor AMDGPUDisassembler

Clean up ahead of a patch to fix bugs in the AMDGPUDisassembler.

Use split-file to simplify and extend existing kernel-descriptor
disassembly tests.

Add a comment to AMDHSAKernelDescriptor.h, as at least one small set
towards keeping all kernel-descriptor sensitive code in sync.

Reviewed By: MaskRay, kzhuravl, arsenm

Differential Revision: https://reviews.llvm.org/D130105