An if that looks like:
if (x) { } else { }
That has no phis following it is dead. Currently these are only
removed by peephole select, but that means that 'x' is considered
used until that pass is run, which can make it difficult to apply
sane lowering in the case where loading 'x' requires complex or
expensive transformations, but 'x' is *really* unused.
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/14400>
c09c0b351f