[IVUsers] Don't bail out of normalizing non-affine add recs

Summary:
In a previous change I changed SCEV's normalization / denormalization
to work with non-affine add recs.  So the bailout in IVUsers can be
removed.

Reviewers: atrick, efriedma

Reviewed By: atrick

Subscribers: davide, mcrosier, llvm-commits

Differential Revision: https://reviews.llvm.org/D32105

llvm-svn: 301298