[InstCombine] Use APInt for all the math in foldICmpDivConstant

Summary: This currently uses ConstantExpr to do its math, but as noted in a TODO it can all be done directly on APInt.

Reviewers: spatel, majnemer

Reviewed By: majnemer

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D38440

llvm-svn: 314640