OpcodeLength in function-body-decoder was implemented in a way that did
not detect invalid non-prefixed opcodes, even when {validate} was on.
This CL brings its behavior in line with prefixed opcodes and validation
requirements.

Change-Id: I53fec32f13bd18a2ed0c7a7666d69fc09603db56
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2552516
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71368}