[LAA] Use re-written SCEV expressions when computing distances

This removes a redundant stride versioning step (we already
do it in getPtrStride, so it has no effect) and uses PSE to
get the SCEV expressions for the source and destination
(this might have changed when getPtrStride was called).

I discovered this through code inspection, and couldn't
produce a regression test for it.

llvm-svn: 269052