[JumpThreading] Don't pass DT to isGuaranteedNotToBeUndefOrPoison()

JumpThreading intentionally does not force updating of the DT
during optimization, because this may be expensive when many CFG
updates and DT calculations are interleaved.

We shouldn't be fetching the DT just for the purpose of calling
isGuaranteedNotToBeUndefOrPoison(), especially as DT availability
doesn't even show benefit in tests.