Wrap the obj and method registers in BuildGetIterator in a register
allocation scope, so that they don't get materialised before the
JumpIfJSReceiver jump if they don't have to.

Bug: v8:9649
Change-Id: I8dfdd06a23c396124c495b5cb83c078080f1a7c9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1768583
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63393}