soft-fp64/fadd: Instead of tracking "b < a", track sign of the difference

soft-fp64/fadd: Instead of tracking "b < a", track sign of the difference

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: 824403 -> 822766 (-0.20%)
instructions in affected programs: 756260 -> 754623 (-0.22%)
helped: 68
HURT: 1
helped stats (abs) min: 1 max: 118 x̄: 26.26 x̃: 18
helped stats (rel) min: 0.02% max: 0.97% x̄: 0.31% x̃: 0.23%
HURT stats (abs)   min: 149 max: 149 x̄: 149.00 x̃: 149
HURT stats (rel)   min: 0.17% max: 0.17% x̄: 0.17% x̃: 0.17%
95% mean confidence interval for instructions value: -31.94 -15.51
95% mean confidence interval for instructions %-change: -0.37% -0.23%
Instructions are helped.

total cycles in shared programs: 6828935 -> 6816791 (-0.18%)
cycles in affected programs: 6385191 -> 6373047 (-0.19%)
helped: 73
HURT: 0
helped stats (abs) min: 2 max: 852 x̄: 166.36 x̃: 120
helped stats (rel) min: <.01% max: 0.80% x̄: 0.22% x̃: 0.17%
95% mean confidence interval for cycles value: -210.80 -121.91
95% mean confidence interval for cycles %-change: -0.27% -0.17%
Cycles are helped.

total fills in shared programs: 1442 -> 1497 (3.81%)
fills in affected programs: 1442 -> 1497 (3.81%)
helped: 0
HURT: 1

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