Fix invalid ptrtoint in InstCombine

Summary:
It's unclear if this is the only thing we can do but at least this is consistent with the check
of address space agreement in `isBitCastable`.

The code is used at least in both instcombine and jumpthreading though
I could only find a way to trigger the invalid cast in instcombine.

Reviewers: loladiro, sanjoy, majnemer

Reviewed By: sanjoy

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D34335

llvm-svn: 316302