Fix crash when indexing unsupported interface blocks by variable 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/631797 Reviewed-by: Olli Etuaho <oetuaho@nvidia.com> Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>