handle merged routes correctly

Signed-off-by: Andres Taylor <andres@planetscale.com>

go/vt/vtgate/planbuilder/operators/route.go

@@ -719,7 +719,7 @@ func (r *Route) GetOrdering(ctx *plancontext.PlanningContext) []OrderBy {
 // in Inputs() and thus not a part of the operator tree
 func (r *Route) TablesUsed(in []string) []string {
  for _, mw := range r.MergedWith {
- in = append(in, mw.TablesUsed(in)...)
+ in = append(in, TablesUsed(mw)...)
  }
  return in
 }