Since slow handler was previously not a Smi. The DCHECK assumed any
Smi Handler on this path should be a proxy handler. Now it Checks for
both, and should continue if the current handler is a slow handler.

Bug: chromium:1008632
Change-Id: I079960894d7320d8d658d0990e8c32db51703206
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1828480Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Suraj Sharma <surshar@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#64052}