glsl: Replace assert with unreachable
glsl: Replace assert with unreachable

Signed-off-by: Ian Romanick <2b237cafb16dc45038e85df6c85e74e6d899eba9@intel.com>
Reviewed-by: Kenneth Graunke <bd2562f754ec92342f93f61c25d731e290a2ffa8@whitecape.org>
Reviewed-by: Eric Engestrom <96f164ad4d9b2b0dacf8ebee2bb1eeb3aa69adf1@engestrom.ch>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler