[flang] Added coarse grained alias analysis for FIR.

These are experimental changes in Flang AA to provide
at least some means to disambiguate memory accesses in some
simple cases. This AA is still not used by any transformation,
so the LIT tests are the only way to trigger it currently.
I will further look into applying this AA within Flang
to address some of the known performance issues in the benchmarks.

Credits to @Renaud-K for the initial implementation.

Differential Revision: https://reviews.llvm.org/D141410