[SLPV] Recognize vectorizable intrinsics during SLP vectorization and
transform accordingly. Based on similar code from Loop vectorization.
Subsequent commits will include vectorization of function calls to
vector intrinsics and form function calls to vector library calls.

Patch by Raul Silvera! (Much delayed due to my not running dcommit)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200576 91177308-0d34-0410-b5e6-96231b3b80d8