StartIndexes Tells exactly which source element is in element 0,
the even source. Nothing needs to be swapped.
Since we're dealing with power of 2 vector lengths, StartIndexes
is almost always even so the condition here was never true. The
exception is when we're interleaving two 1 element vectors. In that
case StartIndexes could be 1.
We recently hit a failure from this on a pulldown. I don't have
the reduced reproducer yet and my naive attempts at making an
interleave of 1 element vectors produces a slideup instead so don't
go through this path.