Refactor lithium classes to reduce the number of virtual functions.

src/x64/lithium-codegen-x64.cc

Index: src/x64/lithium-codegen-x64.cc
===================================================================
--- src/x64/lithium-codegen-x64.cc	(revision 7679)
+++ src/x64/lithium-codegen-x64.cc	(working copy)
@@ -690,7 +690,7 @@
   }
   __ bind(label->label());
   current_block_ = label->block_id();
-  LCodeGen::DoGap(label);
+  DoGap(label);
 }
 
 
@@ -716,6 +716,11 @@
 }
 
 
+void LCodeGen::DoInstructionGap(LInstructionGap* instr) {
+  DoGap(instr);
+}
+
+
 void LCodeGen::DoParameter(LParameter* instr) {
   // Nothing to do.
 }