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/legalizer/split.rs

@@ -125,7 +125,7 @@ fn split_any(
 
     // We have split the value requested, and now we may need to fix some EBB predecessors.
     while let Some(repair) = repairs.pop() {
-        for &(_, inst) in cfg.get_predecessors(repair.ebb) {
+        for (_, inst) in cfg.pred_iter(repair.ebb) {
             let branch_opc = pos.func.dfg[inst].opcode();
             assert!(
                 branch_opc.is_branch(),