Merging r266244:

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r266244 | thomas.stellard | 2016-04-13 13:44:16 -0700 (Wed, 13 Apr 2016) | 13 lines

AMDGPU/SI: Add support for spilling VGPRs without having to scavenge registers

Summary:
When we are spilling SGPRs to scratch memory, we usually don't have
free SGPRs to do the address calculation, so we need to re-use the
ScratchOffset register for the calculation.

Reviewers: arsenm

Subscribers: arsenm, llvm-commits

Differential Revision: http://reviews.llvm.org/D18917

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llvm-svn: 271765