Improve and simplify removal of unreachable code

src/hydrogen.h

Index: src/hydrogen.h
diff --git a/src/hydrogen.h b/src/hydrogen.h
index 004aa16a8784337f874768166b75833c6eea8086..5f85a21afb4657d008fb14699caf7e6fb9e80b53 100644
--- a/src/hydrogen.h
+++ b/src/hydrogen.h
@@ -170,7 +170,11 @@ class HBasicBlock: public ZoneObject {
   HBasicBlock* inlined_entry_block() { return inlined_entry_block_; }
 
   bool IsDeoptimizing() const { return is_deoptimizing_; }
-  void MarkAsDeoptimizing() { is_deoptimizing_ = true; }
+  void MarkAsDeoptimizing();
+
+  void MarkUnreachable();
+  bool IsUnreachable() const { return !is_reachable_; }
+  bool IsReachable() const { return is_reachable_; }
 
   bool IsLoopSuccessorDominator() const {
     return dominates_loop_successors_;
@@ -213,6 +217,7 @@ class HBasicBlock: public ZoneObject {
   HBasicBlock* parent_loop_header_;
   // For blocks marked as inline return target: the block with HEnterInlined.
   HBasicBlock* inlined_entry_block_;
+  bool is_reachable_ : 1;
   bool is_inline_return_target_ : 1;
   bool is_deoptimizing_ : 1;
   bool dominates_loop_successors_ : 1;
@@ -454,6 +459,9 @@ class HGraph: public ZoneObject {
     uint32_instructions_->Add(instr, zone());
   }
 
+  void MarkHasUnreachableCode() { has_unreachable_code_ = true; }
+  bool HasUnreachableCode() const { return has_unreachable_code_; }
+
   void IncrementInNoSideEffectsScope() { no_side_effects_scope_count_++; }
   void DecrementInNoSideEffectsScope() { no_side_effects_scope_count_--; }
   bool IsInsideNoSideEffectsScope() { return no_side_effects_scope_count_ > 0; }
@@ -497,6 +505,7 @@ class HGraph: public ZoneObject {
   CompilationInfo* info_;
   Zone* zone_;
 
+  bool has_unreachable_code_;
   bool is_recursive_;
   bool use_optimistic_licm_;
   bool has_soft_deoptimize_;