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[SLPVectorizer] Try different vectorization factors for store chains

Description

[SLPVectorizer] Try different vectorization factors for store chains
...and set max vector register size based on target

This patch is based on discussion on the llvmdev mailing list:
http://lists.cs.uiuc.edu/pipermail/llvmdev/2015-July/087405.html

and also solves:
https://llvm.org/bugs/show_bug.cgi?id=17170

Several FIXME/TODO items are noted in comments as potential improvements.

Differential Revision: http://reviews.llvm.org/D10950