AMDGPU: Partially fix machine uniformity for inline asm

AMDGPU: Partially fix machine uniformity for inline asm

This was assuming virtual registers only, and asserting on physical.
This was also ignoring AGPRs, and only considering VGPRs.

Reporting the instruction as uniform or not is conceptually wrong,
this should be reported per-operand. An inline asm statement could
include uniform and non-uniform components. This should report
purely for the register defs and ignore the uses.

Fixes asserting on most of the inline asm tests when uniformity
analysis is used.