util: Keep quiet NaNs quiet when converting to half float.

util: Keep quiet NaNs quiet when converting to half float.

We don't want to be throwing exceptions and changing float values later by
emitting a signaling binary16 nan.

If we don't do this, then when we convert back to f32 in NIR constant
expression evaluation, the signaling NaN can end up giving NaN for
fmax(NaN, 0.0), instead of 0.0.

Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/5933
Cc: mesa-stable
Reviewed-by: Emma Anholt <emma@anholt.net>
Reviewed-by: Jason Ekstrand <jason.ekstrand@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16233>