This is apart of the work to change ARC so that all of the dataflow operations
are delegated to visitors which allow for behavior to be specialized depending
on the transformation kind. This will allow for users to add new behavior to the
ARC optimizer will minimally touching the actual dataflow.
Swift SVN r26777
An RCStateTransition is (you guessed it) a transition in the state
of an RCIdentity. It abstracts operations such as:
1. The introduction of a new incremented RCID by an argument (and in
the future
an @owned return value).
2. Strong increment of an RC.
3. Strong decrement of an RC.
I will use this to move pin_removal into the ARC optimizer and will
open up the ARC optimizer to being able to match up unowned values in the
future. It additionally allows me to deduplicate code from
{TopDown,BottomUp}RefCountState into RefCountState, RCStateTransition.
Swift SVN r26608
