nir/flrp: Lower flrp(a, b, c) differently if another flrp(_, b, c) exists

There is little effect on Intel GPUs now because we almost always take
the "always_precise" path first.  It may help on other GPUs, and it does
prevent a bunch of regressions in "intel/compiler: Don't always require
precise lowering of flrp".

No changes on any other Intel platforms.

GM45 and Iron Lake had similar results. (Iron Lake shown)
total cycles in shared programs: 188852500 -> 188852484 (<.01%)
cycles in affected programs: 14612 -> 14596 (-0.11%)
helped: 4
HURT: 0
helped stats (abs) min: 4 max: 4 x̄: 4.00 x̃: 4
helped stats (rel) min: 0.09% max: 0.13% x̄: 0.11% x̃: 0.11%
95% mean confidence interval for cycles value: -4.00 -4.00
95% mean confidence interval for cycles %-change: -0.13% -0.09%
Cycles are helped.

Reviewed-by: Matt Turner <[email protected]>

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