[string] Fast-path in indexOf for subject == search

If the subject string is a string, it's pointer-equal to the search
string, and position <= 0, then we can simply return 0 and skip
all other logic in String.p.indexOf.

Further context at:
https://twitter.com/hashseed/status/893539117367271425

Bug: 
Change-Id: I93ce724f0ade6332599ba395fe8c662a28f05ade
Reviewed-on: https://chromium-review.googlesource.com/602214Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47183}

src/builtins/builtins-string-gen.cc

@@ -793,6 +793,11 @@ void StringBuiltinsAssembler::StringIndexOf(
               &return_minus_1);
   }
 
+  // If the string pointers are identical, we can just return 0. Note that this
+  // implies {start_position} == 0 since we've passed the check above.
+  Label return_zero(this);
+  GotoIf(WordEqual(subject_string, search_string), &return_zero);
+
   // Try to unpack subject and search strings. Bail to runtime if either needs
   // to be flattened.
   ToDirectStringAssembler subject_to_direct(state(), subject_string);
@@ -914,6 +919,9 @@ void StringBuiltinsAssembler::StringIndexOf(
   BIND(&return_minus_1);
   f_return(SmiConstant(-1));
 
+  BIND(&return_zero);
+  f_return(SmiConstant(0));
+
   BIND(&zero_length_needle);
   {
     Comment("0-length search_string");