Document the return value and fix a typo.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@184281 91177308-0d34-0410-b5e6-96231b3b80d8
SLPVectorizer: vectorize compares and selects.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@184282 91177308-0d34-0410-b5e6-96231b3b80d8