[AA] Cache (optionally) estimated PartialAlias offsets.

For the cases of two clobbering loads and one loaded object is fully contained
in the second `BasicAAResult::aliasGEP` returns just `PartialAlias` that
is actually more common case of partial overlap, it doesn't say anything about
actual overlapping sizes.

AA users such as GVN and DSE have no functionality to estimate aliasing of GEPs
with non-constant offsets. The change stores estimated relative offsets so they
can be used further.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D93529