turnip: implement VK_EXT_shader_demote_to_helper_invocation
The "demote" intrinsic has the semantics of D3D discard, which means
it doesn't change the control flow, allowing derivatives to work.
On A6xx there is no known way to check whether invocation was demoted,
thus we use nir_lower_is_helper_invocation.
Add "logical" OPC_DEMOTE which is later translated to "kill".
Such separation is necessary to run "kill" specific optimizations
which are invalid for "demote".
Signed-off-by: Danylo Piliaiev <dpiliaiev@igalia.com>
Reviewed-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9460>
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