AMDGPU: Set StackPointerRegisterToSaveRestore

This will enable selecting non-entry block allocas. Skip the SP write
check in the base isSchedulingBoundary implementation to preserve the
previous scheduling behavior and avoid test churn. It's apparently for
compile time reasons, but if we were to use this more work would be
needed since in some of the failing tests, we seem to incorrectly get
hazard nops inserted.