Remove dead phis as soon as they are discovered.

runtime/vm/compiler.cc

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/compiler.h"
 
 #include "vm/assembler.h"
 
 #include "vm/ast_printer.h"
 #include "vm/code_generator.h"
@@ skipping 163 matching lines @@
 
       FlowGraphOptimizer optimizer(flow_graph);
       optimizer.ApplyICData();
       DEBUG_ASSERT(flow_graph->VerifyUseLists());
 
       // Optimize (a << b) & c patterns. Must occur before
       // 'SelectRepresentations' which inserts conversion nodes.
       // TODO(srdjan): Moved before inlining until environment use list can
       // be used to detect when shift-left is outside the scope of bit-and.
       optimizer.TryOptimizeLeftShiftWithBitAndPattern();
+      DEBUG_ASSERT(flow_graph->VerifyUseLists());
 
       // Inlining (mutates the flow graph)
       if (FLAG_use_inlining) {
         TimerScope timer(FLAG_compiler_stats,
                          &CompilerStats::graphinliner_timer);
         FlowGraphInliner inliner(flow_graph);
         inliner.Inline();
         // Use lists are maintained and validated by the inliner.
         DEBUG_ASSERT(flow_graph->VerifyUseLists());
       }
@@ skipping 453 matching lines @@
       Object::Handle(isolate->object_store()->sticky_error());
     isolate->object_store()->clear_sticky_error();
     isolate->set_long_jump_base(base);
     return result.raw();
   }
   UNREACHABLE();
   return Object::null();
 }
 
 }  // namespace dart