dart2js cps: Global value numbering and loop-invariant code motion.

pkg/compiler/lib/src/cps_ir/loop_hierarchy.dart

Index: pkg/compiler/lib/src/cps_ir/loop_hierarchy.dart
diff --git a/pkg/compiler/lib/src/cps_ir/loop_hierarchy.dart b/pkg/compiler/lib/src/cps_ir/loop_hierarchy.dart
index 3aa5bc9bf64e548bb6373543b869fece700db572..4385eadd2d0d7f37a33b1c8157fc3f38c16725e1 100644
--- a/pkg/compiler/lib/src/cps_ir/loop_hierarchy.dart
+++ b/pkg/compiler/lib/src/cps_ir/loop_hierarchy.dart
@@ -150,4 +150,26 @@ class LoopHierarchy {
     }
     return target;
   }
+
+  /// Returns the the innermost loop that effectively encloses both
+  /// c1 and c2 (or `null` if there is no such loop).
+  Continuation lowestCommonAncestor(Continuation c1, Continuation c2) {
+    int d1 = getDepth(c1), d2 = getDepth(c2);
+    while (c1 != c2) {
+      if (d1 <= d2) {
+        c2 = getEnclosingLoop(c2);
+        d2 = getDepth(c2);
+      } else {
+        c1 = getEnclosingLoop(c1);
+        d1 = getDepth(c1);
+      }
+    }
+    return c1;
+  }
+
+  /// Returns the lexical nesting depth of [loop].
+  int getDepth(Continuation loop) {
+    if (loop == null) return 0;
+    return loopDepth[loop];
+  }
 }