nir: Add a couple of helpers for glsl types.

nir: Add a couple of helpers for glsl types.

This will be used by tgsi_to_nir, which needs to get vec4 types for
declaring shader input/output variables.

v2: Add a missing space.

Reviewed-by: Matt Turner <mattst88@gmail.com> (v2)
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>