This iterator is not actually a SILSuccessorIterator since it is not iterating
over the "successors" of a block. Instead it is used to given the head of a CFG
edge, iterate over the CFG edge's predecessors using the double linked list
stored inside SILSuccessor.
This rename/refactor ties SILSuccessor closer to SILSuccessorIterator and makes
it clear what we are actually iterating over.
This just makes it clearer what this class really does and also makes it clear
that this is really representing a CFGEdge more than a specific successor. I may
rename it to SILCFGEdge at some point to make the name match its usage.
This operator++ is a post-increment operator but it was implemented with pre-increment behavior.
This bug was exposed by the changes in llvm r294339 and caused the master-next branch to stop
building.
A somewhat more natural solution would be to replace the
normal successor with an edge to an unreachable block,
but that would require adding blocks during the iteration.
Add a small amount of SIL infrastructure for asking a
pred_iterator exactly which edge out of a terminator it
represents.
Swift SVN r28514
