BBVectorize: Only some insert element operand pairs are free.

This fixes another infinite recursion case when using target costs.
We can only replace insert element input chains that are pure (end
with inserting into an undef).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@167784 91177308-0d34-0410-b5e6-96231b3b80d8