soft-fp64: Don't open-code umulExtended

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: 928859 -> 859509 (-7.47%)
instructions in affected programs: 866293 -> 796943 (-8.01%)
helped: 76
HURT: 0
helped stats (abs) min: 75 max: 8042 x̄: 912.50 x̃: 688
helped stats (rel) min: 5.35% max: 21.02% x̄: 10.35% x̃: 7.58%
95% mean confidence interval for instructions value: -1138.37 -686.63
95% mean confidence interval for instructions %-change: -11.69% -9.00%
Instructions are helped.

total cycles in shared programs: 7272912 -> 7072275 (-2.76%)
cycles in affected programs: 6763486 -> 6562849 (-2.97%)
helped: 76
HURT: 0
helped stats (abs) min: 214 max: 30136 x̄: 2639.96 x̃: 1923
helped stats (rel) min: 1.75% max: 9.20% x̄: 4.04% x̃: 2.41%
95% mean confidence interval for cycles value: -3455.29 -1824.63
95% mean confidence interval for cycles %-change: -4.69% -3.39%
Cycles are helped.

total spills in shared programs: 817 -> 814 (-0.37%)
spills in affected programs: 791 -> 788 (-0.38%)
helped: 2
HURT: 0

total fills in shared programs: 2438 -> 2488 (2.05%)
fills in affected programs: 2392 -> 2442 (2.09%)
helped: 0
HURT: 2

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

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