This is a preparation to (re-)enable opt_zero_samples(), which will reduce
a SEND mlen before we split it. When that happen, opt_split_sends()
won't be able to rely on the fact that mlen covers the entire
LOAD_PAYLOAD.
Since we are changing that, take the opportunity to also not modify the
existing LOAD_PAYLOAD, just create two new ones with the exact set of
sources. This allows the pass to be further simplified by iterating
forward and not require live_variables analysis.
The helper function was added so can be used later for
opt_zero_samples().
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25742>
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