[x64] Move modrm decoding up the scope

Instead of decoding in each branch, move the decoding up in scope, since
all branches do the same decoding.

Bug: v8:10933
Change-Id: I99ef03e4271cc111428f9a243f61b26af6f85780
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2472758Reviewed-by: Bill Budge <bbudge@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70542}