Computing the length for variable-length TAs is a lot of code and was
regressing microbenchmarks.

Bug: v8:11111
Change-Id: Ia7c3c92bfb43938068aaf539b290f6a30b049c18
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3583898Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Shu-yu Guo <syg@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79961}