AMDGPU: Hack for VS_32 register pressure

AMDGPU: Hack for VS_32 register pressure

For some reason VS_32 ends up factoring into the pressure heuristics
even though we should never see a virtual register with this class.

When SGPRs are reserved for register spilling, this for some reason
triggers reg-crit scheduling.

Setting isAllocatable = 0 may help with this since that seems to remove
it from the default implementation's generated table.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@252321 91177308-0d34-0410-b5e6-96231b3b80d8