[AMDGPU] Make vector superclasses allocatable

The combined vector register classes with both
VGPRs and AGPRs are currently unallocatable.
This patch turns them into allocatable as a
prerequisite to enable copy between VGPR and
AGPR registers during regalloc.

Also, added the missing AV register classes from
192b to 1024b.

Reviewed By: rampitec

Differential Revision: https://reviews.llvm.org/D109300