AMDGPU/SI: Don't try to move scratch wave offset when there are no free SGPRs

Summary:
When there were no free SGPRs, we were trying to move this value into
some of the reserved registers which was causing a segmentation fault.

Reviewers: arsenm

Subscribers: arsenm, llvm-commits

Differential Revision: http://reviews.llvm.org/D17590

llvm-svn: 262577