Remove NaN constant from arith.minf, arith.maxf expansion

If any of the operands is NaN, return the operand instead of a new constant.

When the rhs operand is a constant, the second arith.cmpf+select ops will be folded away.

https://reviews.llvm.org/D117010 marks the two ops commutative, which will place the constant on the rhs.

Reviewed By: herhut

Differential Revision: https://reviews.llvm.org/D117011