Fix a bug where we would corrupt the offset when evaluating

Fix a bug where we would corrupt the offset when evaluating
a non-constant GEP.

I don't have any test case that demonstrates this, Nadav (indirectly)
pointed this out in code review. I'm not sure how possible it is to
contrive a test case for the current users of this code that triggers
the bad issue sadly.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@189188 91177308-0d34-0410-b5e6-96231b3b80d8