nir: Mark bcsel-to-fmin (or fmax) transformations as inexact

nir: Mark bcsel-to-fmin (or fmax) transformations as inexact

These transformations are 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.

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