[AMDGPU] Introduce RC flags for vector register classes

Configure and use the TSFlags in TargetRegisterClass to
have unique flags for VGPR and AGPR register classes.
The vector register class queries like `hasVGPRs` will
now become more efficient with just a bitwise operation.

Reviewed By: rampitec

Differential Revision: https://reviews.llvm.org/D108815