[AMDGPU] Add a regclass flag for scalar registers

Along with vector RC flags, this scalar flag will
make various regclass queries like `isVGPR` more
accurate.

Regclasses other than vectors are currently set
with the new flag even though certain unallocatable
classes aren't truly scalars. It would be ok as long
as they remain unallocatable.

Reviewed By: rampitec

Differential Revision: https://reviews.llvm.org/D110053