AMDGPU: Adjust register allocation priority values down

Set the priorities consistently to number of registers in the tuple -
1. Previously we started at 1, and also tried to give SGPR higher
values than VGPRs. There's no point in assigning SGPRs higher values
now that those are allocated in a separate regalloc run.

This avoids overflowing the 5 bits used for the class priority in the
allocation heuristic for 32 element tuples. This avoids some cases
where smaller registers unexpectedly get prioritized over larger.