[LVI] Refactor to use newly introduced intersect utility

This patch uses the newly introduced 'intersect' utility (from 259461: [LVI] Introduce an intersect operation on lattice values) to simplify existing code in LVI.

While not introducing any new concepts, this change is probably not NFC.  The common 'intersect' function is more powerful that the ad-hoc implementations we'd had in a couple of places.  Given that, we may see optimizations triggering a bit more often.

llvm-svn: 259583