AMDGPU: Add alloc priority to global ranges

The requested register class priorities weren't respected
globally. Not sure why this is a target option, and not just the
expected behavior (recently added in
1a6dc92b). This avoids an allocation
failure when many wide tuple spills are introduced. I think this is a
workaround since I would not expect the allocation priority to be
required, and only a performance hint. The allocator should be smarter
about when only a subregister needs to be spilled and restored.

This does regress a couple of degenerate store stress lit tests which
shouldn't be too important.