Remove duplicated ForOfStatement init code code from RewriteSpreads

Simply call InitializeForOfStatement (split out from InitializeForEachStatement)
instead, which already has all the necessary logic.

As part of this, trade one bool arg (is_destructuring) for an int
(iterable_pos).

Review URL: https://codereview.chromium.org/1740293002

Cr-Commit-Position: refs/heads/master@{#34561}

src/parsing/parser.h

@@ -938,8 +938,10 @@ class Parser : public ParserBase<ParserTraits> {
 
   // Initialize the components of a for-in / for-of statement.
   void InitializeForEachStatement(ForEachStatement* stmt, Expression* each,
-                                  Expression* subject, Statement* body,
-                                  bool is_destructuring);
+                                  Expression* subject, Statement* body);
+  void InitializeForOfStatement(ForOfStatement* stmt, Expression* each,
+                                Expression* iterable, Statement* body,
+                                int iterable_pos);
   Statement* DesugarLexicalBindingsInForStatement(
       Scope* inner_scope, VariableMode mode,
       ZoneList<const AstRawString*>* names, ForStatement* loop, Statement* init,