glsl: Add ir_demote

To represent the new `demote` keyword when using
EXT_demote_to_helper_invocation extension.  Most of the changes are to
include it in the visitors.

Demote is not considered a control flow, so also include an empty
visit member function in ir_control_flow_visitor.

Only NIR actually supports `demote`, so assert the translations for
TGSI and Mesa's gl_program -- since the demote is not expected to
appear for those.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/compiler/glsl/ir_print_visitor.cpp

@@ -564,6 +564,13 @@ ir_print_visitor::visit(ir_discard *ir)
 }
 
 
+void
+ir_print_visitor::visit(ir_demote *ir)
+{
+   fprintf(f, "(demote)");
+}
+
+
 void
 ir_print_visitor::visit(ir_if *ir)
 {