soft-fp64/fadd: Massively split the live range of zFrac0 and zFrac1

soft-fp64/fadd: Massively split the live range of zFrac0 and zFrac1

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: 822766 -> 817327 (-0.66%)
instructions in affected programs: 760943 -> 755504 (-0.71%)
helped: 74
HURT: 0
helped stats (abs) min: 8 max: 515 x̄: 73.50 x̃: 51
helped stats (rel) min: 0.58% max: 1.10% x̄: 0.77% x̃: 0.73%
95% mean confidence interval for instructions value: -91.17 -55.83
95% mean confidence interval for instructions %-change: -0.81% -0.74%
Instructions are helped.

total cycles in shared programs: 6816791 -> 6822826 (0.09%)
cycles in affected programs: 6384809 -> 6390844 (0.09%)
helped: 0
HURT: 74
HURT stats (abs)   min: 6 max: 1179 x̄: 81.55 x̃: 50
HURT stats (rel)   min: 0.02% max: 0.17% x̄: 0.09% x̃: 0.09%
95% mean confidence interval for cycles value: 48.99 114.12
95% mean confidence interval for cycles %-change: 0.09% 0.10%
Cycles are HURT.

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