[SCEV] Factor out common visiting patterns for SCEV rewriters. NFC.

Summary:
Add a SCEVRewriteVisitor class which contains the common
visiting patterns used when rewriting SCEVs.

SCEVParameterRewriter and SCEVApplyRewriter now inherit
from SCEVRewriteVisitor (and are therefore much simpler).

Reviewers: anemet, mzolotukhin, sanjoy

Subscribers: rengolin, llvm-commits, sanjoy

Differential Revision: http://reviews.llvm.org/D13242

llvm-svn: 251283