i965: Structure code so unsupported inst will not generate more errors.
i965: Structure code so unsupported inst will not generate more errors.

We want to rely on brw_opcode_desc() always returning non-NULL in other
validation functions. Other validation functions will be in the else
case of the block added in this patch.

Reviewed-by: Kenneth Graunke <bd2562f754ec92342f93f61c25d731e290a2ffa8@whitecape.org>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler