This is part of a series of patches to evolve ADCE.cpp to support
removing of unnecessary control flow.
This patch updates the propagation of liveness information to handle
special properties of PHI nodes.
We still force all terminators live for now until we add code to
handle removing control flow in a later patch.
No changes to effective behavior with this patch
D23559 [ADCE] Add control dependence computation
D23225 [ADCE] Modify data structures to support removing control flow
D23065 [ADCE] Refactor anticipating new functionality (NFC)
D23102 [ADCE] Refactoring for new functionality (NFC)