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