Merging r196611:

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r196611 | dexonsmith | 2013-12-06 13:48:36 -0800 (Fri, 06 Dec 2013) | 5 lines

Don't use isNullValue to evaluate ConstantExpr

ConstantExpr can evaluate to false even when isNullValue gives false.

Fixes PR18143.
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llvm-svn: 196614