Merging r354144:

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r354144 | spatel | 2019-02-15 17:31:55 +0100 (Fri, 15 Feb 2019) | 3 lines

[InstCombine] fix crash while trying to narrow a binop of shuffles (PR40734)

https://bugs.llvm.org/show_bug.cgi?id=40734
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llvm-svn: 354252