When alias.rt is used to alias certain output components, we might end
up with a situation where some, but not all, of the components of
collects end up being unused. This is currently not supported which
means we end up with useless moves (coming from copy lowering) for
aliased output components.
Fix this by adding support for partial wrmasks for collects in DCE. The
wrmasks are initially zeroed out and then updated based on the wrmask of
their users. Sources of collects for which the corresponding dst ends up
being unused are treated as unused as well. This allows us to remove
the useless output moves by simply updating the wrmask of the end
sources.
Signed-off-by: Job Noorman <jnoorman@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/31222>
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