AMDGPU add support for spilling to a user sgpr pointed buffers

Summary:
This lets you select which sort of spilling you want, either s[0:1] or 64-bit loads from s[0:1].

Patch By: Dave Airlie

Reviewers: nhaehnle, arsenm, tstellarAMD

Reviewed By: arsenm

Subscribers: mareko, llvm-commits, kzhuravl, wdng, yaxunl, tony-tye

Differential Revision: https://reviews.llvm.org/D25428

llvm-svn: 293000