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.h

@@ -1521,8 +1521,19 @@ public:
    /**
     * Optional condition for the assignment.
     */
-   ir_rvalue *condition;
+private:
+   ir_rvalue *unused_condition;
 
+public:
+   inline ir_rvalue *get_condition()
+   {
+      return unused_condition;
+   }
+
+   inline const ir_rvalue *get_condition() const
+   {
+      return unused_condition;
+   }
 
    /**
     * Component mask written