[LVI] Restructure caching

Variant on D70103. The caching is switched to always use a BB to
cache entry map, which then contains per-value caches. A separate
set contains value handles with a deletion callback. This allows us
to properly invalidate overdefined values.

A possible alternative would be to always cache by value first and
have per-BB maps/sets in the each cache entry. In that case we could
use a ValueMap and would avoid the separate value handle set. I went
with the BB indexing at the top level to make it easier to integrate
D69914, but possibly that's not the right choice.

Differential Revision: https://reviews.llvm.org/D70376