Inspired by a commit message in !30934, I set about optimizing the code
generated for nir_copysign. It would be possible to just implement an
opt_algebraic pattern for the specific values used by nir_copysign, but
this casts a slightly larger net.
As noted in a comment in the code, there may be variations of the
pattern that this pass misses. The opt_algebraic pattern would miss them
too.
v2: Use nir_def_replace. Suggested by Alyssa. Allow more "root"
instruction types. Suggested by Georg.
v3: Treat extract_u16(x, 0) as (x & 0x0000ffff), and treat extract_u8(x,
0) as (x & 0x000000ff).
v4: Use nir_scalar. Suggested by Georg.
Reviewed-by: Georg Lehmann <dadschoorse@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/31006>
