[Attributor] Propagate known information from `checkForAllCallSites`

Authored by jdoerfert on Dec 30 2019, 2:12 PM.


[Attributor] Propagate known information from checkForAllCallSites

If we know that all call sites have been processed we can derive an
early fixpoint. The use in this patch is likely not to trigger right now
but a follow up patch will make use of it.

Reviewed By: uenoku, baziotis

Differential Revision: https://reviews.llvm.org/D72016