soft-fp64/fadd: Pick zero or non-zero result based on subtraction result

The main purpose of this commit is to prepare for "soft-fp64/fadd: Move
common code out of both branches of an if-statement".

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: 817327 -> 815717 (-0.20%)
instructions in affected programs: 755504 -> 753894 (-0.21%)
helped: 73
HURT: 1
helped stats (abs) min: 1 max: 159 x̄: 22.12 x̃: 14
helped stats (rel) min: 0.05% max: 0.40% x̄: 0.22% x̃: 0.23%
HURT stats (abs)   min: 5 max: 5 x̄: 5.00 x̃: 5
HURT stats (rel)   min: 0.07% max: 0.07% x̄: 0.07% x̃: 0.07%
95% mean confidence interval for instructions value: -27.27 -16.24
95% mean confidence interval for instructions %-change: -0.24% -0.20%
Instructions are helped.

total cycles in shared programs: 6822826 -> 6820707 (-0.03%)
cycles in affected programs: 6390844 -> 6388725 (-0.03%)
helped: 71
HURT: 3
helped stats (abs) min: 2 max: 537 x̄: 30.72 x̃: 18
helped stats (rel) min: <.01% max: 0.08% x̄: 0.03% x̃: 0.03%
HURT stats (abs)   min: 10 max: 32 x̄: 20.67 x̃: 20
HURT stats (rel)   min: 0.01% max: 0.02% x̄: 0.02% x̃: 0.02%
95% mean confidence interval for cycles value: -43.41 -13.86
95% mean confidence interval for cycles %-change: -0.04% -0.03%
Cycles are helped.

Reviewed-by: Matt Turner <[email protected]>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4142>

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