[regexp] Match spec semantics in AdvanceStringIndex

src/builtins/builtins-regexp.cc

Index: src/builtins/builtins-regexp.cc
diff --git a/src/builtins/builtins-regexp.cc b/src/builtins/builtins-regexp.cc
index fa7241941d7cc90d7a42025b405aa5f664657fd6..d96a9b7ca97fc55a2507a17006e28919db85fa3a 100644
--- a/src/builtins/builtins-regexp.cc
+++ b/src/builtins/builtins-regexp.cc
@@ -1400,8 +1400,8 @@ BUILTIN(RegExpPrototypeSplit) {
                                                  factory->undefined_value()));
 
     if (result->IsNull(isolate)) {
-      string_index += RegExpUtils::AdvanceStringIndex(isolate, string,
-                                                      string_index, unicode);
+      string_index = RegExpUtils::AdvanceStringIndex(isolate, string,
+                                                     string_index, unicode);
       continue;
     }
 
@@ -1416,8 +1416,8 @@ BUILTIN(RegExpPrototypeSplit) {
 
     const int end = std::min(last_index, length);
     if (end == prev_string_index) {
-      string_index += RegExpUtils::AdvanceStringIndex(isolate, string,
-                                                      string_index, unicode);
+      string_index = RegExpUtils::AdvanceStringIndex(isolate, string,
+                                                     string_index, unicode);
       continue;
     }