soft-fp64/fadd: Just let the subtraction happen when the result will be zero

soft-fp64/fadd: Just let the subtraction happen when the result will be zero

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: 815717 -> 815491 (-0.03%)
instructions in affected programs: 735489 -> 735263 (-0.03%)
helped: 39
HURT: 34
helped stats (abs) min: 2 max: 192 x̄: 20.79 x̃: 12
helped stats (rel) min: 0.01% max: 0.46% x̄: 0.26% x̃: 0.28%
HURT stats (abs)   min: 1 max: 65 x̄: 17.21 x̃: 11
HURT stats (rel)   min: <.01% max: 1.11% x̄: 0.35% x̃: 0.19%
95% mean confidence interval for instructions value: -10.40 4.21
95% mean confidence interval for instructions %-change: -0.07% 0.13%
Inconclusive result (value mean confidence interval includes 0).

total cycles in shared programs: 6820707 -> 6813681 (-0.10%)
cycles in affected programs: 6388725 -> 6381699 (-0.11%)
helped: 51
HURT: 23
helped stats (abs) min: 3 max: 1837 x̄: 184.76 x̃: 120
helped stats (rel) min: <.01% max: 0.48% x̄: 0.25% x̃: 0.25%
HURT stats (abs)   min: 18 max: 216 x̄: 104.22 x̃: 98
HURT stats (rel)   min: 0.06% max: 0.73% x̄: 0.31% x̃: 0.11%
95% mean confidence interval for cycles value: -154.67 -35.22
95% mean confidence interval for cycles %-change: -0.15% <.01%
Inconclusive result (%-change mean confidence interval includes 0).

total spills in shared programs: 702 -> 703 (0.14%)
spills in affected programs: 702 -> 703 (0.14%)
helped: 0
HURT: 1

total fills in shared programs: 1497 -> 1499 (0.13%)
fills in affected programs: 1497 -> 1499 (0.13%)
helped: 0
HURT: 1

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