Refactor lithium operand declaration a little bit.

src/lithium-allocator.h

Index: src/lithium-allocator.h
diff --git a/src/lithium-allocator.h b/src/lithium-allocator.h
index 9908ea823d3ed3171a3d7b49f8f2a16f2236db3c..ceebc44abccd25d15922634d34aee95c70c99929 100644
--- a/src/lithium-allocator.h
+++ b/src/lithium-allocator.h
@@ -51,12 +51,9 @@ class LArgument;
 class LPlatformChunk;
 class LOperand;
 class LUnallocated;
-class LConstantOperand;
 class LGap;
 class LParallelMove;
 class LPointerMap;
-class LStackSlot;
-class LRegister;
 
 
 // This class represents a single point of a LOperand's lifetime.