Improve and simplify removal of unreachable code

src/hydrogen-instructions.cc

Index: src/hydrogen-instructions.cc
diff --git a/src/hydrogen-instructions.cc b/src/hydrogen-instructions.cc
index 6b55da09673d096e68971fb9ec83ab1105196a3a..61da776e21eb18b89be428ca1c4fb8b2c2b81c4a 100644
--- a/src/hydrogen-instructions.cc
+++ b/src/hydrogen-instructions.cc
@@ -509,6 +509,17 @@ const char* HValue::Mnemonic() const {
 }
 
 
+bool HValue::CanReplaceWithDummyUses() {
+  return FLAG_unreachable_code_elimination &&
+      !(block()->IsReachable() ||
+        IsBlockEntry() ||
+        IsControlInstruction() ||
+        IsSimulate() ||
+        IsEnterInlined() ||
+        IsLeaveInlined());
+}
+
+
 bool HValue::IsInteger32Constant() {
   return IsConstant() && HConstant::cast(this)->HasInteger32Value();
 }
@@ -1053,6 +1064,21 @@ Representation HBranch::observed_input_representation(int index) {
 }
 
 
+bool HBranch::KnownSuccessorBlock(HBasicBlock** block) {
+  HValue* value = this->value();
+  if (value->EmitAtUses()) {
+    ASSERT(value->IsConstant());
+    ASSERT(!value->representation().IsDouble());
+    *block = HConstant::cast(value)->BooleanValue()
+        ? FirstSuccessor()
+        : SecondSuccessor();
+    return true;
+  }
+  *block = NULL;
+  return false;
+}
+
+
 void HCompareMap::PrintDataTo(StringStream* stream) {
   value()->PrintNameTo(stream);
   stream->Add(" (%p)", *map().handle());
@@ -2860,6 +2886,20 @@ void HCompareObjectEqAndBranch::PrintDataTo(StringStream* stream) {
 }
 
 
+bool HCompareObjectEqAndBranch::KnownSuccessorBlock(HBasicBlock** block) {
+  if (left()->IsConstant() && right()->IsConstant()) {
+    bool comparison_result =
+        HConstant::cast(left())->Equals(HConstant::cast(right()));
+    *block = comparison_result
+        ? FirstSuccessor()
+        : SecondSuccessor();
+    return true;
+  }
+  *block = NULL;
+  return false;
+}
+
+
 void HCompareHoleAndBranch::InferRepresentation(
     HInferRepresentationPhase* h_infer) {
   ChangeRepresentation(value()->representation());