In case a condition with a wrong type is given to a case statement, it
generates an error but the compiler recovers. This caused ValidateSwitch
to assert. Fix this.

BUG=angle:921

Change-Id: I7949798cab923c2b168817471896470c6c611878
Reviewed-on: https://chromium-review.googlesource.com/254080Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>