[Attributor] Delete dead stores

D106185 allows us to determine if a store is needed easily. Using that
knowledge we can start to delete dead stores.

In AAIsDead we now track more state as an instruction can be dead (= the
old optimisitc state) or just "removable". A store instruction can be
removable while being very much alive, e.g., if it stores a constant
into an alloca or internal global. If we would pretend it was dead
instead of only removablewe we would ignore it when we determine what
values a load can see, so that is not what we want.

Differential Revision: https://reviews.llvm.org/D106188