[Attributor] Introduce AAAssumptionInfo to propagate assumptions

This patch introduces a new abstract attributor instance that propagates
assumption information from functions. Conceptually, if a function is
only called by functions that have certain assumptions, then we can
apply the same assumptions to that function. This problem is similar to
calculating the dominator set, but the assumptions are merged instead of
nodes.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D111054