AMDGPU: Use GlobalPriority for largest register tuples

AMDGPU: Use GlobalPriority for largest register tuples

Only do this for 16 and 32 register tuples, although we might want to
extend to 8 tuples.

It's incredibly expensive to spill these, and doing so majorly
interferes with the ability to allocate anything else in the function.

The lit tests show mostly sizeable improvements with a handful of tiny
regressions with large vectors.