InstSimplify: Fold gep X, (sub 0, ptrtoint(X)) to null

Save InstCombine some work if we can perform this fold during
InstSimplify.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216441 91177308-0d34-0410-b5e6-96231b3b80d8