Merging r226588:

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r226588 | adrian | 2015-01-20 10:03:37 -0800 (Tue, 20 Jan 2015) | 1 line

Add an assertion and prefer a crash over an infinite loop.
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git-svn-id: https://llvm.org/svn/llvm-project/llvm/branches/release_36@228984 91177308-0d34-0410-b5e6-96231b3b80d8