[AMDGPU] Allow architected SGPRs for workgroup IDs

Some subtargets use architected SGPRs for workgroup
IDs instead of the regular SGPRs. This patch enables
the support for the same and is guarded under the
subtarget feature FeatureArchitectedSGPRs.

Reviewed By: foad

Differential Revision: https://reviews.llvm.org/D143707