[AMDGPU] On gfx908, reserve VGPR for AGPR copy based on register budget.

Based on available register budget, reserve highest available VGPR for
AGPR copy before RA. After RA, shift it to lowest unused VGPR if the one
exist.

Fixes SWDEV-330006.

Reviewed By: rampitec

Differential Revision: https://reviews.llvm.org/D123525