This patch intends to fix a compiler crash when indexing an
unsupported interface blocks. We should not use UNREACHABLE() here
because the compiler will continue parsing when this kind of error
is generated. Instead, we use an ASSERT to ensure the compile error
must have been reported before when the parsing reaches here.

BUG=chromium:758159

Change-Id: I4bc63316d156d51f721123fe963106d1e81d8d32
Reviewed-on: https://chromium-review.googlesource.com/631797Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>