Add a new constraint corresponding to the AV_* register classes
for operands which can allocate AGPRs or VGPRs. This applies
to load and stores on gfx90a+, and srcA / srcB for MFMA instructions.
The error emitted on unsupported targets isn't ideal, it is
produced by the register allocator without a rationale, but it is
consistent with the existing errors.
I mostly want this for writing allocation tests.
