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>
af1a6f0f77