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/ir_print_visitor.cpp

@@ -444,8 +444,8 @@ void ir_print_visitor::visit(ir_assignment *ir)
 {
    fprintf(f, "(assign ");
 
-   if (ir->condition)
-      ir->condition->accept(this);
+   if (ir->get_condition())
+      ir->get_condition()->accept(this);
 
    char mask[5];
    unsigned j = 0;