Soften assertion in AMDGPU emitPrologue.

[AMDGPU] emitPrologue looks for an unused unallocated SGPR that is not
the scratch descriptor. Continue search if unused register found fails
other requirements.

Reviewers: arsenm, tstellarAMD, nhaehnle

Subscribers: arsenm, llvm-commits, kzhuravl

Differential Revision: http://reviews.llvm.org/D20526

llvm-svn: 270646