Add new target hooks for LoadStoreVectorizer

Summary: Added 6 new target hooks for the vectorizer in order to filter types, handle size constraints and decide how to split chains.

Reviewers: tstellarAMD, arsenm

Subscribers: arsenm, mzolotukhin, wdng, llvm-commits, nhaehnle

Differential Revision: https://reviews.llvm.org/D24727

llvm-svn: 283099