soft-fp64/fadd: Use absolute value of expDiff

soft-fp64/fadd: Use absolute value of expDiff

In one branch we know that expDiff is already positive.

In the other branch we know the expDiff is negative.  Previously in that
branch the code was -(expDiff + 1).  This is equvialent to (-expDiff) -
1, and since expDiff is negative, abs(expDiff) - 1.

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: 818246 -> 819560 (0.16%)
instructions in affected programs: 756423 -> 757737 (0.17%)
helped: 1
HURT: 73
helped stats (abs) min: 1205 max: 1205 x̄: 1205.00 x̃: 1205
helped stats (rel) min: 1.36% max: 1.36% x̄: 1.36% x̃: 1.36%
HURT stats (abs)   min: 2 max: 149 x̄: 34.51 x̃: 27
HURT stats (rel)   min: 0.14% max: 1.09% x̄: 0.41% x̃: 0.30%
95% mean confidence interval for instructions value: -16.56 52.07
95% mean confidence interval for instructions %-change: 0.30% 0.47%
Inconclusive result (value mean confidence interval includes 0).

total cycles in shared programs: 6816686 -> 6817254 (<.01%)
cycles in affected programs: 6384704 -> 6385272 (<.01%)
helped: 37
HURT: 37
helped stats (abs) min: 30 max: 5790 x̄: 289.05 x̃: 102
helped stats (rel) min: 0.04% max: 0.86% x̄: 0.29% x̃: 0.31%
HURT stats (abs)   min: 2 max: 1020 x̄: 304.41 x̃: 232
HURT stats (rel)   min: <.01% max: 1.58% x̄: 0.55% x̃: 0.43%
95% mean confidence interval for cycles value: -165.37 180.72
95% mean confidence interval for cycles %-change: <.01% 0.27%
Inconclusive result (value mean confidence interval includes 0).

total spills in shared programs: 705 -> 591 (-16.17%)
spills in affected programs: 705 -> 591 (-16.17%)
helped: 1
HURT: 0

total fills in shared programs: 1501 -> 1353 (-9.86%)
fills in affected programs: 1501 -> 1353 (-9.86%)
helped: 1
HURT: 0

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