Fix small bug in #36684

PR #36684 changes `iterate(IncrementalCompact)` to return an extra index, but
leaves its arguments unchanged. However, the PR decremented the index argument
in a particular recursive call to `iterate`. This caused `iterate` not to
recognise that it was done when `allow_cfg_transforms` was turned on.

base/compiler/ssair/ir.jl

@@ -1206,7 +1206,7 @@ function iterate(compact::IncrementalCompact, (idx, active_bb)::Tuple{Int, Int}=
         # Move to next block
         compact.idx += 1
         if finish_current_bb!(compact, active_bb, old_result_idx, true)
-            return iterate(compact, (compact.idx-1, active_bb + 1))
+            return iterate(compact, (compact.idx, active_bb + 1))
         else
             return Pair{Pair{Int, Int}, Any}(Pair{Int,Int}(compact.idx-1, old_result_idx), compact.result[old_result_idx][:inst]), (compact.idx, active_bb + 1)
         end