Operations on locals are the most common wasm operations (especially
local.get), and just computing the number of locals from the vector
shows up with a few percent in the profile. Storing this number
separately removes a few percent of execution time from the
{DecodeLocalGet} method.

R=thibaudm@chromium.org

Bug: v8:10576
Change-Id: I9c12842c944e4ad2e602e05838948a33922ff438
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2429950Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70148}