[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
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@326071
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