Reapply "A simpler scheme for garbage collection of ureachable phi inputs."

runtime/vm/flow_graph.cc

Index: runtime/vm/flow_graph.cc
diff --git a/runtime/vm/flow_graph.cc b/runtime/vm/flow_graph.cc
index 65d5de4db6c49fef57641997adce5fff3eae90ae..d434d610ebed6c587bd311253424cd663142f8e5 100644
--- a/runtime/vm/flow_graph.cc
+++ b/runtime/vm/flow_graph.cc
@@ -51,7 +51,6 @@ void FlowGraph::DiscoverBlocks() {
   // Number blocks in reverse postorder.
   intptr_t block_count = postorder_.length();
   for (intptr_t i = 0; i < block_count; ++i) {
-    postorder_[i]->set_block_id(block_count - i - 1);
     reverse_postorder_.Add(postorder_[block_count - i - 1]);
   }
   // Link instructions backwards for optimized compilation.