The current Array#push reduction supports some amount of non-redundant
homogeneous from its receiver maps, and unifies polymorphism by
generating a push implementation per unique receiver elements kind,
rather than per receiver map. It does this by dynamically reading off
the receiver's elements kind, and branching on it.

Reading off the receiver's elements kind dynamically is a bit of a waste
though, since we already know the small subset of maps that are possible
at this point, and have probably emitted diamonds for checking those
maps which can't be merged with the dynamic elements kind lookup.

In this patch, this code is changed in two major ways:

  1. We perform comparisons on the receiver map, rather than the
     receiver elements kind, and dispatch to the per-elements kind
     implementation after that check.

  2. We allow the Smi path to fallthrough into the Object elements path,
     once its Smi checks complete, to avoid generating distinct but
     identical grow-and-set code for both PACKED_ELEMENTS and
     PACKED_SMI_ELEMENTS.

Change-Id: Ie7764339a0220cb30aee0592553e0dc98539ac79
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3912765Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/main@{#83438}