SLP Vectorizer:  Add support for trees with external users.

To support this we have to insert 'extractelement' instructions to pick the right lane.
We had this functionality before but I removed it when we moved to the multi-block design because it was too complicated.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185230 91177308-0d34-0410-b5e6-96231b3b80d8