This is similar to D76853. Current implementation of division estimation isn't correct for 1.0/0.0 (result is nan, not expected inf).
And this change exposes a potential infinite loop: in combineRepeatedFPDivisors, we use isConstOrConstSplatFP to look up if the divisor is some constant. But it doesn't work after legalized on some platforms. This patch restricts the method to act before LegalDAG.