Promote the BasicBlock tuple to a real struct;

It makes reading code that uses it easier to understand.

lib/codegen/src/verifier/cssa.rs

@@ -2,7 +2,7 @@
 
 use dbg::DisplayList;
 use dominator_tree::{DominatorTree, DominatorTreePreorder};
-use flowgraph::ControlFlowGraph;
+use flowgraph::{BasicBlock, ControlFlowGraph};
 use ir::{ExpandedProgramPoint, Function};
 use regalloc::liveness::Liveness;
 use regalloc::virtregs::VirtRegs;
@@ -138,7 +138,7 @@ impl<'a> CssaVerifier<'a> {
     fn check_cssa(&self) -> VerifierResult<()> {
         for ebb in self.func.layout.ebbs() {
             let ebb_params = self.func.dfg.ebb_params(ebb);
-            for (_, pred) in self.cfg.pred_iter(ebb) {
+            for BasicBlock { inst: pred, .. } in self.cfg.pred_iter(ebb) {
                 let pred_args = self.func.dfg.inst_variable_args(pred);
                 // This should have been caught by an earlier verifier pass.
                 assert_eq!(