read_prefixed_byte is used mostly to read an entire prefixed opcode, it
writes the number of bytes of the opcode index (without prefix byte) to
the out param length. Change it so it writes the total number of bytes
(including the prefix byte), as that is what most callers want (they add
1 after calling read_prefixed_byte).

Bug: v8:10810
Change-Id: I914190ecae62e3547652accdc05d1cef3686fff4
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2476678Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70551}