[Attributor] Use CFG reasoning to filter potentially interfering writes

[Attributor] Use CFG reasoning to filter potentially interfering writes

Since D104432 we can look through memory by analyzing all writes that
might interfere with a load. This patch provides some logic to exclude
writes that cannot interfere with a location, due to CFG reasoning.
We make sure to avoid multi-thread write-read situations properly while
we ignore writes that cannot reach a load or writes that will be
overwritten before the load is reached.

Differential Revision: https://reviews.llvm.org/D106397