[MemDep] Use BatchAA when computing pointer dependencies

We're not changing IR while running a single MemDep query, so it's
safe to cache alias analysis results using BatchAA. This adds BatchAA
usage to getSimplePointerDependencyFrom(), which is non-intrusive --
covering larger parts (like a whole processNonLocalLoad query) is
also possible, but requires threading BatchAA through a bunch of APIs.

For the ThinLTO configuration, this is a 1% geomean improvement on CTMark.

Differential Revision: https://reviews.llvm.org/D85583