Restructure RegExp exec cache code.

src/string.js

Index: src/string.js
diff --git a/src/string.js b/src/string.js
index 30eedb3c9a2fe9451ccf08a5d0469ae91b3143d3..d97f632b88dc32975830ecc2f50186199c1768ce 100644
--- a/src/string.js
+++ b/src/string.js
@@ -611,7 +611,7 @@ function StringSplit(separator, limit) {
   if (%_ObjectEquals(cache.type, 'split') &&
       %_IsRegExpEquivalent(cache.regExp, separator) &&
       %_ObjectEquals(cache.subject, subject) &&
-      %_ObjectEquals(cache.lastIndex, limit)) {
+      %_ObjectEquals(cache.splitLimit, limit)) {
     if (cache.answerSaved) {
       return CloneDenseArray(cache.answer);
     } else {
@@ -622,8 +622,7 @@ function StringSplit(separator, limit) {
   cache.type = 'split';
   cache.regExp = separator;
   cache.subject = subject;
-  // Reuse lastIndex field for split limit when type is "split".
-  cache.lastIndex = limit;
+  cache.splitLimit = limit;
 
   %_Log('regexp', 'regexp-split,%0S,%1r', [subject, separator]);