[MemorySSA] Use BatchAA for clobber walker

While MemorySSA use optimization was already using BatchAA, the
publicly exposed MSSA walkers were using plain AAResults. This is
not great, because it is expected that clobber walking will make
repeated AA queries.

This patch makes the clobber API accept a BatchAAResults instance.
The plain APIs are kept as wrappers and will create a BatchAAResults
instance for the duration of the query. In the future, the explicit
BatchAAResults arguments will be used to share AA results across
queries, not just within one query.

Differential Revision: https://reviews.llvm.org/D136164