Fix assert with GEP ptr vector indexing structs

Also fix it calculating the wrong value. The struct index
is not a ConstantInt, so it was being interpreted as an array
index.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188713 91177308-0d34-0410-b5e6-96231b3b80d8