[TTI] getMinMaxReductionCost - add FastMathFlag argument

Similar to the getArithmeticReductionCost / getExtendedReductionCost calls (which really don't need to use std::optional<>).

This will be necessary to correct recognize fast/nnan fmax/fmul reductions which can avoid nan handling - which will allow us to remove the fmax/fmin special case in X86TTIImpl::getMinMaxCost and use getIntrinsicInstrCost like we do for integer reductions (63c38953).

Differential Revision: https://reviews.llvm.org/D148149