Revert "Revert two GEP-related InstCombine commits"

This reverts commit r216698 which reverted r216523 and r216598.

We would attempt to perform the transformation even if the match()
failed because, as a side effect, it would set V.  This would trick us
into believing that we correctly found a place to correctly apply the
transform.

An additional test case was added to getelementptr.ll so that we might
not regress in the future.