swift234

Improve epilogue retain matcher.

Instead of only checking the return block, we could potentially check
its predecessors and its predecessors's predecessors, etc.

Alos put in a threshold to throttle this to make sure its cheap.

We are still only being able to find of a small # of epilogue retains.
The bail on MayDecrement is blocking many of the opportunites.

This should bring us closer to being able to handle Walsh.

This is part of rdar://24022375.