Fix representation inference for mutable double boxes.

test/mjsunit/regress/regress-3307.js

Index: test/mjsunit/regress/regress-3307.js
diff --git a/test/mjsunit/regress/regress-check-eliminate-loop-phis.js b/test/mjsunit/regress/regress-3307.js
similarity index 54%
copy from test/mjsunit/regress/regress-check-eliminate-loop-phis.js
copy to test/mjsunit/regress/regress-3307.js
index 3791c35f71ccfc50dd384978e449103b39f6bff7..eba16cd7b9c3d8e456eed1a89e09dfcc8f07831b 100644
--- a/test/mjsunit/regress/regress-check-eliminate-loop-phis.js
+++ b/test/mjsunit/regress/regress-3307.js
@@ -4,18 +4,21 @@
 
 // Flags: --allow-natives-syntax
 
+function p(x) {
+  this.x = x;
+}
+
 function f() {
-  var o = {x:1};
-  var y = {y:2.5, x:0};
-  var result;
-  for (var i = 0; i < 2; i++) {
-    result = o.x + 3;
-    o = y;
+  var a = new p(1), b = new p(2);
+  for (var i = 0; i < 1; i++) {
+    a.x += b.x;
   }
-  return result;
+  return a.x;
 }
 
-f();
-f();
+new p(0.1); // make 'x' mutable box double field in p.
+
+assertEquals(3, f());
+assertEquals(3, f());
 %OptimizeFunctionOnNextCall(f);
 assertEquals(3, f());