nir: Recognize some more open-coded fmin / fmax

nir: Recognize some more open-coded fmin / fmax

This transformation is inexact because section 4.7.1 (Range and
Precision) says:

    Operations and built-in functions that operate on a NaN are not
    required to return a NaN as the result.

The fmin or fmax might not return NaN in cases where the original
expression would be required to return NaN.

v2: Reorder operands and mark as inexact.  The latter suggested by
Jason.

shader-db results:

Haswell, Broadwell, and Skylake had similar results. (Skylake shown)
total instructions in shared programs: 14514817 -> 14514808 (<.01%)
instructions in affected programs: 229 -> 220 (-3.93%)
helped: 3
HURT: 0
helped stats (abs) min: 1 max: 4 x̄: 3.00 x̃: 4
helped stats (rel) min: 2.86% max: 4.12% x̄: 3.70% x̃: 4.12%

total cycles in shared programs: 533145211 -> 533144939 (<.01%)
cycles in affected programs: 37268 -> 36996 (-0.73%)
helped: 8
HURT: 0
helped stats (abs) min: 2 max: 134 x̄: 34.00 x̃: 2
helped stats (rel) min: 0.02% max: 14.22% x̄: 3.53% x̃: 0.05%

Sandy Bridge and Ivy Bridge had similar results. (Ivy Bridge shown)
total cycles in shared programs: 257618409 -> 257618403 (<.01%)
cycles in affected programs: 12582 -> 12576 (-0.05%)
helped: 3
HURT: 0
helped stats (abs) min: 2 max: 2 x̄: 2.00 x̃: 2
helped stats (rel) min: 0.05% max: 0.05% x̄: 0.05% x̃: 0.05%

No changes on Iron Lake or GM45.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>