[SCEVExpander] Try harder to avoid introducing inttoptr

Summary:
This fixes introduction of an incorrect inttoptr/ptrtoint pair in
the included test case which makes use of non-integral pointers. I
suspect there are more cases like this left, but this takes care of
the one I was seeing at the moment.

Reviewers: sanjoy

Subscribers: mzolotukhin, llvm-commits

Differential Revision: https://reviews.llvm.org/D33129

llvm-svn: 304058