ScopInfo: Check for possibly nested GEP in fixed-size delin

We currently only consider the first GEP when delinearizing access functions,
which makes us loose information about additional index expression offsets,
which results in our SCoP model to be incorrect. With this patch we now
compare the base pointers used to ensure we do not miss any additional offsets.
This fixes llvm.org/PR27195.

We may consider supporting nested GEP in our delinearization heuristics in
the future.

llvm-svn: 265379