[SCEV] Introduce SCEVPostIncRewriter

The patch introduces the SCEVPostIncRewriter rewriter which
is similar to SCEVInitRewriter but rewrites AddRec with post increment
value of this AddRec.

This is a preparation for re-writing isKnownPredicate utility as
described in https://reviews.llvm.org/D42417.

Reviewers: sanjoy, mkazantsev, reames
Reviewed By: sanjoy
Subscribers: llvm-commits
Differential Revision: https://reviews.llvm.org/D43499

llvm-svn: 326071