We've been hitting asserts in this code due to the many supported
combintions of modes (iv-rewrite/no-iv-rewrite) and IV types. This
second rewrite of the code attempts to deal with these cases systematically.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@143546 91177308-0d34-0410-b5e6-96231b3b80d8