Remove unused CalleeSavedRegs defs CSR_AMDGPU_VGPRs_24_255 and
CSR_AMDGPU_VGPRs_32_255 which appear to be leftovers from old iterations
of the calling convention.
Rename CalleeSavedRegs defs to avoid being overly specific:
- CSR_AMDGPU_AGPRs_32_255 => CSR_AMDGPU_AGPRs
- CSR_AMDGPU_SGPRs_32_105 => CSR_AMDGPU_SGPRs
- CSR_AMDGPU_HighRegs => CSR_AMDGPU
- CSR_AMDGPU_HighRegs_With_AGPRs => CSR_AMDGPU_GFX90AInsts
Introduce a class RegMask to mark the cases where we use the
CalleeSavedRegs class purely as an expedient way to produce a mask.
Other targets also seem to do this, so a reasonable alternative is to
actually update table-gen to include a new class to do this explicitly,
but the current approach seems harmless so I opted to just make it more