i965/nir/vec4: Implement store_output intrinsic

i965/nir/vec4: Implement store_output intrinsic

This implementation is based on the current URB setup in vec4_visitor, which
requires the output register to be stored in the output_reg array at variable's
original shader location index. But since nir_lower_io() pass uses the value
in var->data.driver_location, we need to put there var->data.location instead,
prior to calling nir_lower_io(), so that we end up with the correct index
in const_index[0].

The driver_location is not used at all, so this patch also disables the
nir_assign_var_locations pass on non-scalar shaders.

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>