[SCEV] Consider delinearization pattern with extension with identity factor

Summary: The delinearization algorithm did not consider terms which had an extension without a multiply factor, i.e. a identify factor. We lose cases where size is char type where there will no multiply factor.

Reviewers: sanjoy, grosser

Subscribers: mzolotukhin, Eugene.Zelenko, llvm-commits, mssimpso, sanjoy, grosser

Differential Revision: https://reviews.llvm.org/D16492

llvm-svn: 284378