Merge BBVectorizer changes r167731, r167743, r167750, r167784, r167811, r167817.

These changes fix a serious interaction problem with the cost model on x86 that
could cause the vectorizer to enter an infinite loop (and sometimes crash in
other ways).


git-svn-id: https://llvm.org/svn/llvm-project/llvm/branches/release_32@167993 91177308-0d34-0410-b5e6-96231b3b80d8