glsl: Make ir_assignment::condition private

And add get_condition().

This proof that nothing remains that could possibly set ::condition to
anything other than NULL.

v2: Fix bad rebase.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/14573>

src/compiler/glsl/opt_array_splitting.cpp

@@ -416,7 +416,7 @@ ir_array_splitting_visitor::visit_leave(ir_assignment *ir)
             new(mem_ctx) ir_dereference_array(ir->rhs->clone(mem_ctx, NULL),
                                               new(mem_ctx) ir_constant(i));
 
-         assert(ir->condition == NULL);
+         assert(ir->get_condition() == NULL);
 
          ir_assignment *assign_i = new(mem_ctx) ir_assignment(lhs_i, rhs_i);