[AliasSetTracker] More precise AAInfo intersection check

The code currently checks whether the intersection has one of TBAA,
Scope or NoAlias unset -- however, those might have already been
unset in the first place, in which case we will unnecessarily
report a change. Instead, compare the intersection result to the
original AAInfo.

This makes for a 0.5% geomean compile-time saving on CTMark.

Differential Revision: https://reviews.llvm.org/D83430