[IndVars] Split `WidenIV::cloneIVUser`; NFC

Summary:
This NFC splitting is intended to make a later diff easier to follow.
It just tail duplicates `cloneIVUser` into `cloneArithmeticIVUser` and
`cloneBitwiseIVUser`.

Reviewers: atrick, hfinkel, reames

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D13716

llvm-svn: 250481