Apparently, 45 mins is legal for international to international connections at HND. The airlines should be able to tell you if they will interline your luggage... Even so, I wouldn't risk 90 minute connection on separate PNRs. If you're delayed by an hour at SIN, you could be SOL. I don't think it's worth the risk. YMMV.

EDIT: jerry a. laska has posted the legal connection time for SQ to DL connection from Expert Flyer in his post below.