Currently the documented API for AnalyzeBranch leaves no place for
conditional returns. This leaves a blindspot after shrinkwrapping on
platforms with conditional return instructions.
The existing API can be used to handle conditional returns by using a null TBB
and a non-empty condition list. A null TBB should never mean fallthrough with a
non-empty condition list, as that wouldn't make any sense. This leaves a
natural way to represent conditional return branches.
Next up is to add support to PPC for AnalyzeBranch and conditional