Handle no-postdominator case in finalizer pass

This pass was assuming that the post-dominator of all finalizer uses
exists as a real BB in the CFG.

base/compiler/ssair/passes.jl

@@ -1614,6 +1614,7 @@ function try_resolve_finalizer!(ir::IRCode, idx::Int, finalizer_idx::Int, defuse
     end
     all(check_defuse, defuse.uses) || return nothing
     all(check_defuse, defuse.defs) || return nothing
+    bb_insert_block != 0 || return nothing # there is no post-dominator of all uses
 
     # Check #3
     dominates(domtree, finalizer_bb, bb_insert_block) || return nothing