For a cast with src and destination size being unequal, we were costing
the cast as if it were being scalarized, when in fact we can often
promote such cases to a wider legal type.
Note that for casts with equal size (i.e. bitcast, some fp<->i, and
ptrtoint) the generic logic in BasicTTI already assumed promotion. It
just doesn't handle the cast where source and destination are both
promoted to non-equal types.
This is analogous to d3fd28a, but with the same reasoning applied to
casts instead.
ac6e1fd0c0