For wide bytecodes, save the bytecode offset as the offset of the prefix
bytecode, rather than the bytecode itself. This means that any code that reads
the bytecode can explicitly know the width of the bytecode at the offset
without having to iterate through the complete bytecode array.

Also simplifies some code in the bytecode analysis that had to work around
the previous approach.

BUG=chromium:753705

Change-Id: I8a42e7cfff27791e39f3452e2b9e52c0608d28cb
Reviewed-on: https://chromium-review.googlesource.com/634003
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47599}