i965/vec4: Allow cmod propagation when src0 is a uniform or shader input

i965/vec4: Allow cmod propagation when src0 is a uniform or shader input

No shader-db changes.  This source must have been written by a previous
instruction, so it cannot be a uniform or a shader input.  However, this
change allows the next commit to help more shaders.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>