soft-fp64/fadd: Common code optimization for differing sign case

soft-fp64/fadd: Common code optimization for differing sign case

This is basically the same ideas from the previous 4 commits applied
to the aSign != bSign part... and all smashed into one commit.

The shader hurt for spill and / or fills is from
KHR-GL46.gpu_shader_fp64.builtin.inverse_dmat4.

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

Tiger Lake
total instructions in shared programs: 787258 -> 683638 (-13.16%)
instructions in affected programs: 725435 -> 621815 (-14.28%)
helped: 74
HURT: 0
helped stats (abs) min: 152 max: 10261 x̄: 1400.27 x̃: 975
helped stats (rel) min: 11.61% max: 20.92% x̄: 15.40% x̃: 14.86%
95% mean confidence interval for instructions value: -1740.11 -1060.43
95% mean confidence interval for instructions %-change: -16.01% -14.79%
Instructions are helped.

total cycles in shared programs: 6483227 -> 5458858 (-15.80%)
cycles in affected programs: 6051245 -> 5026876 (-16.93%)
helped: 74
HURT: 0
helped stats (abs) min: 1566 max: 95474 x̄: 13842.82 x̃: 9757
helped stats (rel) min: 13.94% max: 23.26% x̄: 17.98% x̃: 17.57%
95% mean confidence interval for cycles value: -17104.25 -10581.40
95% mean confidence interval for cycles %-change: -18.61% -17.35%
Cycles are helped.

total spills in shared programs: 553 -> 445 (-19.53%)
spills in affected programs: 553 -> 445 (-19.53%)
helped: 1
HURT: 0

total fills in shared programs: 1307 -> 1323 (1.22%)
fills in affected programs: 1307 -> 1323 (1.22%)
helped: 0
HURT: 1

Ice Lake
total instructions in shared programs: 781216 -> 678470 (-13.15%)
instructions in affected programs: 720088 -> 617342 (-14.27%)
helped: 74
HURT: 0
helped stats (abs) min: 153 max: 8863 x̄: 1388.46 x̃: 975
helped stats (rel) min: 11.24% max: 21.03% x̄: 15.47% x̃: 15.01%
95% mean confidence interval for instructions value: -1703.57 -1073.35
95% mean confidence interval for instructions %-change: -16.09% -14.85%
Instructions are helped.

total cycles in shared programs: 6464085 -> 5453997 (-15.63%)
cycles in affected programs: 6031771 -> 5021683 (-16.75%)
helped: 74
HURT: 0
helped stats (abs) min: 1552 max: 90317 x̄: 13649.84 x̃: 9650
helped stats (rel) min: 13.84% max: 23.11% x̄: 17.83% x̃: 17.41%
95% mean confidence interval for cycles value: -16802.89 -10496.79
95% mean confidence interval for cycles %-change: -18.46% -17.21%
Cycles are helped.

total spills in shared programs: 279 -> 368 (31.90%)
spills in affected programs: 279 -> 368 (31.90%)
helped: 0
HURT: 1

total fills in shared programs: 973 -> 1155 (18.71%)
fills in affected programs: 973 -> 1155 (18.71%)
helped: 0
HURT: 1

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