Expose CFG predecessors only as an iterator.

Define two public iterator types in the flowgraph module, PredIter and
SuccIter, which are by-value iterators over an EBB's predecessors and
successors respectively.

Provide matching pred_iter() and succ_iter() methods for inspecting the
CFG. Remove the get_predecessors() method which returned a slice.

Update the uses of get_predecessors(), none of which depended on it
being a slice.

This abstraction makes it possible to change the internal representation
of the CFG.

lib/cretonne/src/verifier/flags.rs

@@ -60,7 +60,7 @@ impl<'a> FlagsVerifier<'a> {
                     // Revisit any predecessor blocks the first time we see a live-in for `ebb`.
                     None => {
                         self.livein[ebb] = value.into();
-                        for &(pred, _) in self.cfg.get_predecessors(ebb) {
+                        for (pred, _) in self.cfg.pred_iter(ebb) {
                             worklist.insert(pred);
                         }
                     }