VNInfo Question

In LLVM 3.1, given a VNInfo, how do I get the range information for that
value? I've tried looking at the MachineInstr at the def point, getting
its defined register and looking at LiveIntervals to get an interval for
the register, but that doesn't always return something sane. I'm in the
middle of InlineSpiller so LiveRangeEdit is in play.

Thanks for your help!