AMDPGU/R600: Replace llvm_unreachable() call with LLVMContext::emitError()

Summary:
This fixes an issue on MIPS where the infinite-loop-evergreen.ll test
was failing to terminate.

Fixes PR24147.

Reviewers: arsenm, dsanders

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D11260

llvm-svn: 242410