Merging r278786:

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r278786 | jamesm | 2016-08-16 02:45:36 -0700 (Tue, 16 Aug 2016) | 4 lines

Left shifts of negative values are defined if -fwrapv is set

This means we shouldn't emit ubsan detection code or warn.
Fixes PR25552.
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llvm-svn: 278989