[VectorCombine] Add a mask for SK_Broadcast shuffle costing (#85808)

This is part of a series of small patches to compute shuffle masks for
the couple of cases where we call getShuffleCost without one. My goal is
to add an invariant that all calls to getShuffleCost for fixed length
vectors have a mask.

Note that this code appears to be reachable with scalable vectors, and
thus we have to only pass a non-empty mask when the number of elements
is precisely known.