Improve and simplify removal of unreachable code

src/x64/lithium-codegen-x64.cc

Index: src/x64/lithium-codegen-x64.cc
diff --git a/src/x64/lithium-codegen-x64.cc b/src/x64/lithium-codegen-x64.cc
index abb8c77b5b772cd0440251034944771b8277abd9..adc3e18c5e3fa2f3f8f30275d76457519b3b9d10 100644
--- a/src/x64/lithium-codegen-x64.cc
+++ b/src/x64/lithium-codegen-x64.cc
@@ -271,16 +271,27 @@ bool LCodeGen::GenerateBody() {
     }
     if (!emit_instructions) continue;
 
-    if (FLAG_code_comments && instr->HasInterestingComment(this)) {
-      Comment(";;; <@%d,#%d> %s",
-              current_instruction_,
-              instr->hydrogen_value()->id(),
-              instr->Mnemonic());
-    }
+    if (instr->hydrogen_value()->block()->IsReachable()) {
+      if (FLAG_code_comments && instr->HasInterestingComment(this)) {
+        Comment(";;; <@%d,#%d> %s",
+                current_instruction_,
+                instr->hydrogen_value()->id(),
+                instr->Mnemonic());
+      }
 
-    RecordAndUpdatePosition(instr->position());
+      RecordAndUpdatePosition(instr->position());
 
-    instr->CompileToNative(this);
+      instr->CompileToNative(this);
+    } else {
+      if (FLAG_code_comments && instr->IsLabel()) {
+        Comment(
+            ";;; <@%d,#%d> -------------------- B%d (unreachable/elided) "
+            "--------------------",
+            current_instruction_,
+            instr->hydrogen_value()->id(),
+            instr->hydrogen_value()->block()->block_id());
+      }
+    }
   }
   EnsureSpaceForLazyDeopt(Deoptimizer::patch_size());
   return !is_aborted();
@@ -1872,6 +1883,7 @@ void LCodeGen::DoArithmeticT(LArithmeticT* instr) {
 
 int LCodeGen::GetNextEmittedBlock() const {
   for (int i = current_block_ + 1; i < graph()->blocks()->length(); ++i) {
+    if (!graph()->blocks()->at(i)->IsReachable()) continue;
     if (!chunk_->GetLabel(i)->HasReplacement()) return i;
   }
   return -1;