[turbofan] Move a special case out of ReduceElementAccess

...to make things easier to read.

R=jarin@chromium.org

Change-Id: I0e53ef67e34f696b5977d4e091c7bc7bdf0ec145
Reviewed-on: https://chromium-review.googlesource.com/c/1477739Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59706}

src/compiler/js-native-context-specialization.h

@@ -118,6 +118,9 @@ class V8_EXPORT_PRIVATE JSNativeContextSpecialization final
   Reduction ReduceKeyedLoadFromHeapConstant(Node* node, Node* index,
                                             FeedbackNexus const& nexus,
                                             KeyedAccessLoadMode load_mode);
+  Reduction ReduceElementAccessOnString(Node* node, Node* index, Node* value,
+                                        AccessMode access_mode,
+                                        KeyedAccessLoadMode load_mode);
 
   Reduction ReduceSoftDeoptimize(Node* node, DeoptimizeReason reason);
   Reduction ReduceJSToString(Node* node);