SLPVectorizer: refactor the code that places extracts. Place the code that decides where to put extracts in the build-tree phase. This allows us to take the cost of the extracts into account.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186058 91177308-0d34-0410-b5e6-96231b3b80d8