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

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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@276980 91177308-0d34-0410-b5e6-96231b3b80d8