soft-fp64/fadd: Rename aFrac and bFrac variables

soft-fp64/fadd: Rename aFrac and bFrac variables

Exchanging aFracHi / bFracHi and aFracLo / bFracLo should not affect the
result of the later call to __add64.

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

v2: Fix a typo in a comment.  Noticed by Matt.

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: 812094 -> 818246 (0.76%)
instructions in affected programs: 750271 -> 756423 (0.82%)
helped: 0
HURT: 74
HURT stats (abs)   min: 7 max: 520 x̄: 83.14 x̃: 59
HURT stats (rel)   min: 0.52% max: 1.48% x̄: 0.89% x̃: 0.84%
95% mean confidence interval for instructions value: 63.96 102.31
95% mean confidence interval for instructions %-change: 0.83% 0.95%
Instructions are HURT.

total cycles in shared programs: 6797157 -> 6816686 (0.29%)
cycles in affected programs: 6365175 -> 6384704 (0.31%)
helped: 0
HURT: 74
HURT stats (abs)   min: 16 max: 1690 x̄: 263.91 x̃: 181
HURT stats (rel)   min: 0.14% max: 0.68% x̄: 0.32% x̃: 0.27%
95% mean confidence interval for cycles value: 199.74 328.07
95% mean confidence interval for cycles %-change: 0.29% 0.36%
Cycles are HURT.

total spills in shared programs: 703 -> 705 (0.28%)
spills in affected programs: 703 -> 705 (0.28%)
helped: 0
HURT: 1

total fills in shared programs: 1499 -> 1501 (0.13%)
fills in affected programs: 1499 -> 1501 (0.13%)
helped: 0
HURT: 1

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