nir_types: add glsl_type_is_leaf helper

nir_types: add glsl_type_is_leaf helper

Helper used to know when a glsl_type is a leaf when iteraring through
a complex type. Note that GLSL IR linking also uses the concept of
leaf while doing the same iteration, although in that case it uses a
visitor. See link_uniform_blocks, process_array_leaf and others as
reference.

Signed-off-by: Alejandro Piñeiro <apinheiro@igalia.com>
Signed-off-by: Antia Puentes <apuentes@igalia.com>
v2:
   * Moved from gl_nir_linker to nir_types, so it could be used on nir
     xfb gathering (Timothy Arceri)
   * Minor update after Timothy's series about record to struct
     renaming landed master.

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>