Fix re-initialization of existing double field.

test/mjsunit/regress/regress-2717.js

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-// Flags: --allow-natives-syntax
+// Test re-initializing existing field which is already being tracked as
+// having double representation.
+(function() {
+  function test1(a) {
+    return { x: 1.5, x: a };
+  };
 
-function boom() {
-  var a = {
-    foo: "bar",
-    foo: "baz"
+  assertEquals({}, test1({}).x);
+})();
+
+// Test initializing new field which follows an existing transition to a
+// map that tracks it as having double representation.
+(function() {
+  function test1(a) {
+    return { y: a };
   };
-  return a;
-}
 
-assertEquals("baz", boom().foo);
-assertEquals("baz", boom().foo);
-%OptimizeFunctionOnNextCall(boom);
-assertEquals("baz", boom().foo);
+  function test2(a) {
+    return { y: a };
+  };
+
+  assertEquals(1.5, test1(1.5).y);
+  assertEquals({}, test2({}).y);
+})();