Fix handling of overflow in loop calculation by adding new UDiv SCEV. This SCEV

Fix handling of overflow in loop calculation by adding new UDiv SCEV. This SCEV
is disabled in the sense that it will refuse to create one from a UDiv
instruction, until the code is better tested.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@44163 91177308-0d34-0410-b5e6-96231b3b80d8