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_clone.cpp

@@ -102,6 +102,12 @@ ir_discard::clone(void *mem_ctx, struct hash_table *ht) const
    return new(mem_ctx) ir_discard(new_condition);
 }
 
+ir_demote *
+ir_demote::clone(void *mem_ctx, struct hash_table *ht) const
+{
+   return new(mem_ctx) ir_demote();
+}
+
 ir_loop_jump *
 ir_loop_jump::clone(void *mem_ctx, struct hash_table *ht) const
 {