For mode = kArrayFunction, we know that we need to call the array
constructor stub, so we don't need to read it out from the construct_stub
field. We also don't need to set it in the construct_stub field anymore,
so just use the builtins constructor stub like other builtins.

Also cleans up PushArgsThenCall by adding a dcheck that we are never in
mode = kArrayFunction, so we don't even try to generate code for this
case, but fail earlier instead.

We don't need to load the array function in ArrayConstructor because this
is set up for us by the builtins construct stub. We do have to check if
new_target is actually set before overwriting it with target, as we are
handling both call and construct cases in ArrayConstructor now.

Bug: v8:7503
Change-Id: I3622bf6127eebed8b55c9c199fa938a8e03b8baa
Reviewed-on: https://chromium-review.googlesource.com/973364
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52152}