Fix for a smi stores optimization on x64 with a test case.

test/mjsunit/compiler/smi-stores-opt.js

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 // Flags: --allow-natives-syntax
 
+var o = {a:1.5};
+o.a = 0;
+var a = o.a;
+
 function g() {
-  this.x = this;
+  return 1;
 }
 
+var o2 = {a:{}};
+
 function f() {
-  return new g();
+  var result = {a: a};
+  var literal = {x:g()};
+  return [result, literal];
 }
 
 f();
 f();
 %OptimizeFunctionOnNextCall(f);
-f();
+assertEquals(1, f()[1].x);