[SCEV] Clean up isKnownPredicateViaConstantRanges; NFCI

 - ScalarEvolution::isKnownPredicateViaConstantRanges duplicates some
   logic already present in ConstantRange, use ConstantRange for those
   bits.

 - In some cases ScalarEvolution::isKnownPredicateViaConstantRanges
   returns `false` to mean "definitely false" (e.g. see the
   `LHSRange.getSignedMin().sge(RHSRange.getSignedMax())` case for
   `ICmpInst::ICMP_SLT`), but for `isKnownPredicateViaConstantRanges`,
   `false` actually means "don't know".  Get rid of this extra bit of
   code to avoid confusion.

llvm-svn: 259401