Reapply "SLPVectorizer: Handle more horizontal reductions (disabled)""

Reapply r191108 with a fix for a memory corruption error I introduced.  Of
course, we can't reference the scalars that we replace by vectorizing and then
call their eraseFromParent method. I only 'needed' the scalars to get the
DebugLoc. Just store the DebugLoc before actually vectorizing instead. As a nice
side effect, this also simplifies the interface between BoUpSLP and the
HorizontalReduction class to returning a value pointer (the vectorized tree
root).

radar://14607682

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@191123 91177308-0d34-0410-b5e6-96231b3b80d8