This addresses a TODO in JSTypedLowering and generally makes the more
easier to follow since the methods deal only with one kind of Node now.

Bug: v8:8015
Change-Id: I8c3521b8d630dbe272264dc01e9ab3a5b0a8f682
Reviewed-on: https://chromium-review.googlesource.com/1196883Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55530}