Reapply "SLPVectorizer: Ignore users that are insertelements we can reschedule them"

This commit reapplies 205018. After 205855 we should correctly vectorize
intrinsics.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205965 91177308-0d34-0410-b5e6-96231b3b80d8