Merging r242410:

Merging r242410:
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r242410 | tstellar | 2015-07-16 08:38:29 -0700 (Thu, 16 Jul 2015) | 13 lines

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
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git-svn-id: https://llvm.org/svn/llvm-project/llvm/branches/release_37@242453 91177308-0d34-0410-b5e6-96231b3b80d8