[AMDGPU] Pick available high VGPR for CSR SGPR spilling (#78669)

CSR SGPR spilling currently uses the early available physical VGPRs. It
currently imposes a high register pressure while trying to allocate
large VGPR tuples within the default register budget.

This patch changes the spilling strategy by picking the VGPRs in the
reverse order, the highest available VGPR first and later after regalloc
shift them back to the lowest available range. With that, the initial
VGPRs would be available for allocation and possibility
of finding large number of contiguous registers will be more.