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

This reverts commit r205018.

Conflicts:
	lib/Transforms/Vectorize/SLPVectorizer.cpp
	test/Transforms/SLPVectorizer/X86/insert-element-build-vector.ll

This is breaking libclc build.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205260 91177308-0d34-0410-b5e6-96231b3b80d8