[mlir] Propagate NaNs in PolynomialApproximation

Previously, NaNs would be dropped in favor of bounded values which was
strictly incorrect. Now the min/max operation propagate this
information. Not all uses of min/max need this, but the given change
will help protect future additions, and this prevents the need for an
additional cmpf and select operation to handle NaNs.

Differential Revision: https://reviews.llvm.org/D120020