compact: Don't try to kill the same edge twice (#48343)

In IR like:

```
goto if not true
goto if not true
```

our implementation of `kill_edge!` goes through recursively
to kill all newly unreachable blocks. However, it was still
attempting to schedule the newly unreachable block. Then,
when it got to the next GotoIfNot, it wsa again attempting
to kill the same edge, which would fail, because the edge
had already been removed from the CFG. Fix that by
telling IncrementalCompact not to attempt scheduling
any blocks that were newly discovered to be dead.

base/compiler/ssair/ir.jl

@@ -1189,6 +1189,12 @@ function kill_edge!(compact::IncrementalCompact, active_bb::Int, from::Int, to::
                 compact.result[stmt][:inst] = nothing
             end
             compact.result[last(stmts)][:inst] = ReturnNode()
+        else
+            # Tell compaction to not schedule this block. A value of -2 here
+            # indicates that the block is not to be scheduled, but there should
+            # still be an (unreachable) BB inserted into the final IR to avoid
+            # disturbing the BB numbering.
+            compact.bb_rename_succ[to] = -2
         end
     else
         # Remove this edge from all phi nodes in `to` block
@@ -1531,7 +1537,7 @@ function iterate_compact(compact::IncrementalCompact)
         resize!(compact, old_result_idx)
     end
     bb = compact.ir.cfg.blocks[active_bb]
-    if compact.cfg_transforms_enabled && active_bb > 1 && active_bb <= length(compact.bb_rename_succ) && compact.bb_rename_succ[active_bb] == -1
+    if compact.cfg_transforms_enabled && active_bb > 1 && active_bb <= length(compact.bb_rename_succ) && compact.bb_rename_succ[active_bb] <= -1
         # Dead block, so kill the entire block.
         compact.idx = last(bb.stmts)
         # Pop any remaining insertion nodes