Previosly, LoadArrayElement assumed that the array is a FixedArray.
The PropertyArray and WeakFixedArray pretended to be a FixedArray and
had static asserts about length offsets.

This patch make LoadArrayElement generic and uses a new LoadArrayLength
function to fetch the length of the array without hard-coding the length
offset.

Bug: v8:8486
Change-Id: Ib27132bf3fcecc135ad632c4227c57ca0a05036f
Reviewed-on: https://chromium-review.googlesource.com/c/1346498
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57723}