Smis are represented in feedback by the heap number map. Turns out the
megamorphic load IC always just missed instead of using the map. Fixing this
improves a microbenchmark by ~6x.

Change-Id: I4c6d0ec1abb88ae34b212cfc69cc6b020060753b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3683318Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80923}