[AMDGPU] Consolidate SGPRSpill and VGPRSpill into single Spill bit (#81901)

Follow on to #81525 in the series of consolidating bits in TSFlags.

Merge SGPRSpill and VGPRSpill into single Spill bit

Modify isSGPRSpill and isVGPRSpill helper functions to differentiate
VGPR and SGPR spills:

Spill+SALU=SGPR Spill
Spill+VALU=VGPR Spill

The only exception here is SGPR spills to VGPRs which require an
explicit instruction check.