Removing unused HIR opcodes.

Progress on #291.

src/xenia/cpu/hir/opcodes.h

@@ -200,10 +200,7 @@ enum Opcode {
   OPCODE_SWIZZLE,
   OPCODE_PACK,
   OPCODE_UNPACK,
-  OPCODE_COMPARE_EXCHANGE,
   OPCODE_ATOMIC_EXCHANGE,
-  OPCODE_ATOMIC_ADD,
-  OPCODE_ATOMIC_SUB,
   __OPCODE_MAX_VALUE,  // Keep at end.
 };