An enum-typed value should never have a value outside of that enum's
range.
This patch enforces that in Debug mode, while in Release mode keeping
the previous behavior of returning "UnknownOpcode" as the mnemonic for
illegal IrOpcode values to ease debugging.

Bug: v8:3770
Change-Id: I83a5a356f1fb7a266921940a4495f1d39a1823cd
Reviewed-on: https://chromium-review.googlesource.com/c/1436221Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59102}