We previously disabled this under fast math due to aggressive
reassociation by the machine combiner. But I think we can work
around this by using a FSUB instead of FADD for the first
This matches the similar algorithm we do for uint_to_fp i64->f64
in TargetLowering::expandUINT_TO_FP. If reassociation hasn't
been a problem for that, hopefully its not a problem here.