Refactored code a bit to improve StringReplace performance

src/string.js

Index: src/string.js
===================================================================
--- src/string.js	(revision 15496)
+++ src/string.js	(working copy)
@@ -184,8 +184,10 @@
   var subject = TO_STRING_INLINE(this);
   if (IS_REGEXP(regexp)) {
     // Emulate RegExp.prototype.exec's side effect in step 5, even though
-    // value is discarded.
-    ToInteger(regexp.lastIndex);
+    // value is discarded. Inline ToInteger() manually.
+    if (! %_IsSmi(regexp.lastIndex)) {
+      ToNumber(regexp.lastIndex);
+    }
     if (!regexp.global) return RegExpExecNoTests(regexp, subject, 0);
     %_Log('regexp', 'regexp-match,%0S,%1r', [subject, regexp]);
     // lastMatchInfo is defined in regexp.js.
@@ -235,8 +237,10 @@
 
   if (IS_REGEXP(search)) {
     // Emulate RegExp.prototype.exec's side effect in step 5, even if
-    // value is discarded.
-    ToInteger(search.lastIndex);
+    // value is discarded. Inline ToInteger() manually.
+    if (! %_IsSmi(search.lastIndex)) {
+      ToNumber(search.lastIndex);
+    }
     %_Log('regexp', 'regexp-replace,%0r,%1S', [search, subject]);
 
     if (!IS_SPEC_FUNCTION(replace)) {