ScalarEvolution: Introduce SCEVSDivision and SCEVUDivision

It turns out that not all users of SCEVDivision want the same
signedness.  Let the users determine which operation they'd like by
explicitly choosing SCEVUDivision or SCEVSDivision.

findArrayDimensions and computeAccessFunctions will use SCEVSDivision
while HowFarToZero will use SCEVUDivision.