glsl: remove glsl_type::get_vec4_type()

Thanks to Kenneth Graunke for pointing out that glsl_type::get_instance(base, 4, 1)
is the same as glsl_type::get_vec4_type(base).

The function was only used in st_glsl_to_tgsi, and this commit replaces that usage
with get_instance.
2 files changed