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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222104 91177308-0d34-0410-b5e6-96231b3b80d8
Convert some EVTs to MVTs where only a SimpleValueType is needed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222109 91177308-0d34-0410-b5e6-96231b3b80d8
Fix typo

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222110 91177308-0d34-0410-b5e6-96231b3b80d8