soft-fp64: Pick a single idiom for treating sign value as a Boolean

soft-fp64: Pick a single idiom for treating sign value as a Boolean

Replace all of the bool(qSign) with qSign != 0u.  Remove unnecessary
parenthesis from around most of the existing qSign != 0u.

This dramatically simplifies the next commit.

Results on the 308 shaders extracted from the fp64 portion of the OpenGL
CTS:

Tiger Lake and Ice Lake had similar results. (Tiger Lake shown)
total instructions in shared programs: 848109 -> 848106 (<.01%)
instructions in affected programs: 53 -> 50 (-5.66%)
helped: 1
HURT: 0

total cycles in shared programs: 6969145 -> 6969125 (<.01%)
cycles in affected programs: 396 -> 376 (-5.05%)
helped: 1
HURT: 0

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4142>