We already have nir_imm_float16 and nir_imm_vec4; let's add the ability to easily make immediate fp16 vectors as well, now that fp16 support is maturing in NIR/GLSL. Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com> Reviewed-by: Matt Turner <mattst88@gmail.com>