SLPVectorizer: Change the order in which new instructions are added to the function.

SLPVectorizer: Change the order in which new instructions are added to the function.
We are not working on a DAG and I ran into a number of problems when I enabled the vectorizations of 'diamond-trees' (trees that share leafs).
* Imroved the numbering API.
* Changed the placement of new instructions to the last root.
* Fixed a bug with external tree users with non-zero lane.
* Fixed a bug in the placement of in-tree users.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@182508 91177308-0d34-0410-b5e6-96231b3b80d8