[DAG] FoldConstantVectorArithmetic - remove SDNodeFlags argument

Another minor step towards merging FoldConstantVectorArithmetic into FoldConstantArithmetic.

We don't use SDNodeFlags in any constant folding inside DAG, so passing the Flags argument is a waste of time - an alternative would be to wire up FoldConstantArithmetic to take SDNodeFlags just-in-case we someday start using it, but we don't have any way to test it and I'd prefer to avoid dead code.

Differential Revision: https://reviews.llvm.org/D113276