[turbofan] Respect ConsString invariant.

test/mjsunit/regress/regress-crbug-654723.js

Index: test/mjsunit/regress/regress-crbug-654723.js
diff --git a/test/mjsunit/regress/regress-crbug-629435.js b/test/mjsunit/regress/regress-crbug-654723.js
similarity index 68%
copy from test/mjsunit/regress/regress-crbug-629435.js
copy to test/mjsunit/regress/regress-crbug-654723.js
index b73f601c71fcf9fac7a321da8648d056dbe1edd4..fa812335223306d9aa8ce508d7ac6463e750523a 100644
--- a/test/mjsunit/regress/regress-crbug-629435.js
+++ b/test/mjsunit/regress/regress-crbug-654723.js
@@ -4,16 +4,13 @@
 
 // Flags: --allow-natives-syntax
 
-function bar(v) {
-  v.constructor;
-}
-
-bar([]);
-bar([]);
+var k = "0101010101010101" + "01010101";
 
-function foo() {
-  var x = -0;
-  bar(x + 1);
+function foo(s) {
+  return k + s;
 }
+
+foo("a");
+foo("a");
 %OptimizeFunctionOnNextCall(foo);
-foo();
+var x = foo("");