dart2js cps: Add logical rewriter rules to favor null-aware operators.

These useful patterns arise with null-aware operators:

x ? y : x  ==>  x && y
x ? x : y  ==>  x || y

BUG=
[email protected]

Review URL: https://codereview.chromium.org//1291073002 .

Author: asgerf

pkg/compiler/lib/src/tree_ir/optimization/logical_rewriter.dart

@@ -306,6 +306,15 @@ class LogicalRewriter extends RecursiveTransformer
       node.elseExpression = tmp;
     }
 
+    // x ? y : x ==> x && y
+    if (isSameVariable(node.condition, node.elseExpression)) {
+      return new LogicalOperator.and(node.condition, node.thenExpression);
+    }
+    // x ? x : y ==> x || y
+    if (isSameVariable(node.condition, node.thenExpression)) {
+      return new LogicalOperator.or(node.condition, node.elseExpression);
+    }
+
     return node;
   }
 
@@ -468,6 +477,16 @@ class LogicalRewriter extends RecursiveTransformer
         e.elseExpression = (e.elseExpression as Not).operand;
         return new Not(e);
       }
+
+      // x ? y : x ==> x && y
+      if (isSameVariable(e.condition, e.elseExpression)) {
+        return new LogicalOperator.and(e.condition, e.thenExpression);
+      }
+      // x ? x : y ==> x || y
+      if (isSameVariable(e.condition, e.thenExpression)) {
+        return new LogicalOperator.or(e.condition, e.elseExpression);
+      }
+
       return e;
     }
     if (e is Constant && e.value.isBool) {
@@ -509,5 +528,19 @@ class LogicalRewriter extends RecursiveTransformer
       return new LogicalOperator.or(e1, e2);
     }
   }
+
+  /// True if [e2] is known to return the same value as [e1] 
+  /// (with no additional side effects) if evaluated immediately after [e1].
+  /// 
+  /// Concretely, this is true if [e1] and [e2] are uses of the same variable,
+  /// or if [e2] is a use of a variable assigned by [e1].
+  bool isSameVariable(Expression e1, Expression e2) {
+    if (e1 is VariableUse) {
+      return e2 is VariableUse && e1.variable == e2.variable;
+    } else if (e1 is Assign) {
+      return e2 is VariableUse && e1.variable == e2.variable;
+    }
+    return false;
+  }
 }