[AMDGPU] Split R600 and GCN subregs
These are generated and do not need to have the same values.
We are defining separate subregs for R600 and GCN but then
using AMDGPU subregs on R600.
Differential Revision: https://reviews.llvm.org/D74248
rampitec on Feb 7 2020, 12:08 PM.Authored by