This reorders arguments in preparation for removing ebx from its
calling convention (in a follow-up some args will be passed on the
stack).

Drive-by: Improve readability in the code handling different cases
(array,spread,...).

Bug: v8:6666
Change-Id: I0160f8efafd0fd0e841739578e01c32b38adb66e
Reviewed-on: https://chromium-review.googlesource.com/1196884Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55557}