Improve and simplify removal of unreachable code

src/x64/lithium-x64.h

Index: src/x64/lithium-x64.h
diff --git a/src/x64/lithium-x64.h b/src/x64/lithium-x64.h
index 77bebe64bd718c43d6c7f60f2a441e7f22b32b7c..197c5dae5a6f857356a0200819e3a36f40ab5244 100644
--- a/src/x64/lithium-x64.h
+++ b/src/x64/lithium-x64.h
@@ -2519,6 +2519,11 @@ class LChunkBuilder BASE_EMBEDDED {
   // Build the sequence for the graph.
   LPlatformChunk* Build();
 
+  LInstruction* ElideControlInstruction(HControlInstruction* instr,
+                                        bool condition);
+
+  void PropagateUnreachableBlockMarks();
+
   // Declare methods that deal with the individual node types.
 #define DECLARE_DO(type) LInstruction* Do##type(H##type* node);
   HYDROGEN_CONCRETE_INSTRUCTION_LIST(DECLARE_DO)