adjust branch names

IsXXX  with xxx as condition, same as after if_xxx
AlwaysBranch is back to Branch

lib/register/instructions/branch.rb

@@ -16,16 +16,16 @@ module Register
     alias :inspect :to_s
   end
 
-  class IsZeroBranch < Branch
+  class IsZero < Branch
   end
 
-  class IsNegativeBranch < Branch
+  class IsNotzero < Branch
   end
 
-  class IsPositiveBranch < Branch
+  class IsMinus < Branch
   end
 
-  class AlwaysBranch < Branch
+  class IsPlus < Branch
   end
 
 end