AMDGPU/SI: Add support for spiling SGPRs to scratch buffer

Summary:
This is necessary for when we run out of VGPRs and can no
longer use v_{read,write}_lane for spilling SGPRs.

Reviewers: arsenm

Subscribers: arsenm, llvm-commits

Differential Revision: http://reviews.llvm.org/D17592

llvm-svn: 262732