soft-fp64: Store sign value as 0 or 0x80000000

soft-fp64: Store sign value as 0 or 0x80000000

...instead of 0 or 1.  Many places the sign bit is extracted, then later
put back in the same position.  This saves some left-shift operations.

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: 848106 -> 844558 (-0.42%)
instructions in affected programs: 833480 -> 829932 (-0.43%)
helped: 106
HURT: 1
helped stats (abs) min: 1 max: 995 x̄: 33.48 x̃: 12
helped stats (rel) min: 0.15% max: 2.20% x̄: 0.60% x̃: 0.35%
HURT stats (abs)   min: 1 max: 1 x̄: 1.00 x̃: 1
HURT stats (rel)   min: <.01% max: <.01% x̄: <.01% x̃: <.01%
95% mean confidence interval for instructions value: -51.88 -14.43
95% mean confidence interval for instructions %-change: -0.71% -0.47%
Instructions are helped.

total cycles in shared programs: 6969125 -> 6962024 (-0.10%)
cycles in affected programs: 6717689 -> 6710588 (-0.11%)
helped: 78
HURT: 7
helped stats (abs) min: 2 max: 2083 x̄: 110.27 x̃: 56
helped stats (rel) min: <.01% max: 0.30% x̄: 0.11% x̃: 0.11%
HURT stats (abs)   min: 2 max: 1340 x̄: 214.29 x̃: 4
HURT stats (rel)   min: 0.01% max: 0.71% x̄: 0.13% x̃: 0.02%
95% mean confidence interval for cycles value: -144.02 -23.06
95% mean confidence interval for cycles %-change: -0.12% -0.07%
Cycles are helped.

total spills in shared programs: 814 -> 759 (-6.76%)
spills in affected programs: 814 -> 759 (-6.76%)
helped: 2
HURT: 1

total fills in shared programs: 2488 -> 2412 (-3.05%)
fills in affected programs: 2488 -> 2412 (-3.05%)
helped: 2
HURT: 1

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>