partr: fix performance trap from #44653

See issue #44720

Author: vtjnash

base/partr.jl

@@ -65,7 +65,7 @@ function sift_down(heap::taskheap, idx::Int32)
     if idx <= heap.ntasks
         for child = (heap_d * idx - heap_d + Int32(2)):(heap_d * idx + Int32(1))
             child > tasks_per_heap && break
-            if isassigned(heap.tasks, child) &&
+            if isassigned(heap.tasks, Int(child)) &&
                     heap.tasks[child].priority < heap.tasks[idx].priority
                 t = heap.tasks[idx]
                 heap.tasks[idx] = heap.tasks[child]