[AST] Restrict AliasSetTracker to immutable IR

This restricts usage of AliasSetTracker to IR that does not change.
We used to use it during LICM where the underlying IR could change,
but remaining uses all use AST as part of a separate analysis phase.

This is split out from D137955, which makes use of the new guarantee
to switch to BatchAA.

Differential Revision: https://reviews.llvm.org/D138014