AMDGPU/SI: Use v_readfirstlane_b32 when restoring SGPRs spilled to scratch

We were using v_readlane_b32 with the lane set to zero, but this won't
work if thread 0 is not active.

Differential Revision: http://reviews.llvm.org/D19745

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@268295 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed