[turbofan] Mark side-effect-free calls to string ops as kEliminatable.

test/mjsunit/compiler/regress-string-to-number-add.js

Index: test/mjsunit/compiler/regress-string-to-number-add.js
diff --git a/test/mjsunit/compiler/regress-dead-throw-inlining.js b/test/mjsunit/compiler/regress-string-to-number-add.js
similarity index 56%
copy from test/mjsunit/compiler/regress-dead-throw-inlining.js
copy to test/mjsunit/compiler/regress-string-to-number-add.js
index 097a20bc41d34760931928c45e4117888295f20a..e872401c0bba58885b691a358ea2f447e4d1c9ea 100644
--- a/test/mjsunit/compiler/regress-dead-throw-inlining.js
+++ b/test/mjsunit/compiler/regress-string-to-number-add.js
@@ -2,12 +2,14 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// Flags: --allow-natives-syntax
+// Flags: --allow-natives-syntax --turbo-type-feedback
 
-function g() { if (false) throw 0; }
-function f() { g(); }
+function f(x) {
+  var s = x ? "0" : "1";
+  return 1 + Number(s);
+}
 
-f();
-f();
+f(0);
+f(0);
 %OptimizeFunctionOnNextCall(f);
-f();
+assertEquals(2, f(0));