Merging r281650:

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r281650 | david.majnemer | 2016-09-15 13:10:09 -0700 (Thu, 15 Sep 2016) | 6 lines

[InstCombine] Do not RAUW a constant GEP

canRewriteGEPAsOffset expects to process instructions, not constants.

This fixes PR30342.

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llvm-svn: 288066