So far, we decoded instructions with the 0xFB prefix as two-byte, i.e.
a single "u8" byte following the prefix.
This patch changes that to 0xFB + LEB, which is how all prefixed
instructions are supposed to do it. Currently this makes a difference
only for the stringref proposal (instructions 0x80 through 0xb3).

It has the unfortunate consequence that all stringref instructions need
three bytes for now. We expect them to go back to a two-byte encoding
scheme (while remaining LEB compliant) when their final encoding is
decided.

Bug: v8:12868
Change-Id: I603f60adae88e9b985cb65288d9eeb7f98da8138
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3825887
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82400}