[ValueTracking] Fix crash in GetPointerBaseWithConstantOffset()

Summary:
While walking defs of pointer operands we were assuming that the pointer
size would remain constant.  This is not true, because addresspacecast
instructions may cast the pointer to an address space with a different
pointer width.

This partial reverts r282612, which was a more conservative solution
to this problem.

Reviewers: reames, sanjoy, apilipenko

Subscribers: wdng, llvm-commits

Differential Revision: https://reviews.llvm.org/D24772

llvm-svn: 283557