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

This reverts commit r191108.

The horizontal.ll test case fails under libgmalloc. Thanks Shuxin for pointing
this out to me.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@191121 91177308-0d34-0410-b5e6-96231b3b80d8