Merging r276980:

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r276980 | tstellar | 2016-07-28 07:30:43 -0700 (Thu, 28 Jul 2016) | 12 lines

AMDGPU/SI: Don't use reserved VGPRs for SGPR spilling

Summary:
We were using reserved VGPRs for SGPR spilling and this was causing
some programs with a workgroup size of 1024 to use more than 64
registers, which is illegal.

Reviewers: arsenm, mareko, nhaehnle

Subscribers: nhaehnle, arsenm, llvm-commits, kzhuravl

Differential Revision: https://reviews.llvm.org/D22032
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llvm-svn: 277084