This gets rid of all the RewriteNonPattern gunk in the parser and
expression classifier, and removes one use of RewritableExpression.

This borrows pieces from several other CLs of mine which are currently
open, and includes a new and modernized abstraction for dealing with
iterators in BytecodeGenerator (so, this CL adds that, moves code from
BuildGetIterator around, and makes some minor changes to yield* which
should maintain compatability with the old behaviour).

This also implements a portion of the changes to the iteration protocol
(implemented fully in
https://chromium-review.googlesource.com/c/v8/v8/+/687997), but only for
the spread operator in Array Literals (the rest will follow).

BUG=v8:5940, v8:3018
R=rmcilroy@chromium.org, marja@chromium.org, adamk@chromium.org
TBR=adamk@chromium.org

Change-Id: Ifc494d663d8e46066a439c3541c33f0243726234
Reviewed-on: https://chromium-review.googlesource.com/804396
Commit-Queue: Caitlin Potter <caitp@igalia.com>
Reviewed-by: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Mythri Alle <mythria@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50138}