glsl: remove element_type() helper

We now have is_array() and without_array() that make the
code much clearer and remove the need for this.

For all remaining calls to this we already knew that
the type was an array so returning a null wasn't adding any value.

v2: use without_array() in _mesa_ast_array_index_to_hir() and don't use
 without_array() in lower_clip_distance_visitor() as we want to make sure the
 array is 2D.

Reviewed-by: Matt Turner <mattst88@gmail.com>

src/glsl/ast_array_index.cpp

@@ -225,7 +225,7 @@ _mesa_ast_array_index_to_hir(void *mem_ctx,
        * values *do* diverge, then the behavior of the operation requiring a
        * dynamically uniform expression is undefined.
        */
-      if (array->type->element_type()->is_sampler()) {
+      if (array->type->without_array()->is_sampler()) {
 	 if (!state->is_version(130, 100)) {
 	    if (state->es_shader) {
 	       _mesa_glsl_warning(&loc, state,