Creates a macro that in debug builds generates case statements for
unused bytes codes (marked UNREACHABLE). This will catch the case where
a byte code declared to be unused is actually used. Should be easier to
maintain than the existing comments.

Change-Id: I0b5d830be88b7ef747975657283c1b1e98182360
Reviewed-on: https://chromium-review.googlesource.com/928650Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51482}