ir: Give ir_instruction a print visitor helper.

This avoids spamming each file with includes of ir_print_visitor.h
because someone was doing debugging at some point, and is less typing
when doing debugging.

ir_constant_variable.cpp

@@ -33,7 +33,6 @@
  */
 
 #include "ir.h"
-#include "ir_print_visitor.h"
 #include "ir_visitor.h"
 #include "ir_optimization.h"
 #include "glsl_types.h"