Introduce NUMBER_IS_FINITE macro to avoid ToNumber() conversion.

src/string.js

Index: src/string.js
===================================================================
--- src/string.js	(revision 6109)
+++ src/string.js	(working copy)
@@ -127,7 +127,7 @@
   var index = subLength - patLength;
   if (%_ArgumentsLength() > 1) {
     var position = ToNumber(%_Arguments(1));
-    if (!$isNaN(position)) {
+    if (!NUMBER_IS_NAN(position)) {
       position = TO_INTEGER(position);
       if (position < 0) {
         position = 0;