[SCEV] Memoize visitMulExpr results in SCEVRewriteVisitor.

Summary:
When SCEVRewriteVisitor traverses the SCEV DAG, it may visit the same SCEV
multiple times if this SCEV is referenced by multiple other SCEVs. This has
exponential time complexity in the worst case. Memoizing the results will
avoid re-visiting the same SCEV. Add a map to save the results, and override
the visit function of SCEVVisitor. Now SCEVRewriteVisitor only visit each
SCEV once and thus returns the same result for the same input SCEV.

This patch fixes PR18606, PR18607.

Reviewers: Sanjoy Das, Mehdi Amini, Michael Zolotukhin

Differential Revision: https://reviews.llvm.org/D25810

llvm-svn: 284868