BBVectorize: Use a more sophisticated check for input cost

The old checking code, which assumed that input shuffles and insert-elements
could always be folded (and thus were free) is too simple.
This can only happen in special circumstances.
Using the simple check caused infinite recursion.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@167750 91177308-0d34-0410-b5e6-96231b3b80d8