Introduce lhs and rhs aliases to assignment nodes for consistency.

The methods are already available for `masgn`, so this will allow all assignment node types to have the same two methods so that they can be dealt with in unison.

lib/rubocop/ast/node/asgn_node.rb

@@ -12,13 +12,15 @@ class AsgnNode < Node
       def name
         node_parts[0]
       end
+      alias lhs name
 
       # The expression being assigned to the variable.
       #
       # @return [Node] the expression being assigned.
       def expression
         node_parts[1]
       end
+      alias rhs expression
     end
   end
 end

lib/rubocop/ast/node/casgn_node.rb

@@ -9,13 +9,15 @@ class CasgnNode < Node
       include ConstantNode
 
       alias name short_name
+      alias lhs short_name
 
       # The expression being assigned to the variable.
       #
       # @return [Node] the expression being assigned.
       def expression
         node_parts[2]
       end
+      alias rhs expression
     end
   end
 end

lib/rubocop/ast/node/op_asgn_node.rb

@@ -10,6 +10,7 @@ class OpAsgnNode < Node
       def assignment_node
         node_parts[0]
       end
+      alias lhs assignment_node
 
       # The name of the variable being assigned as a symbol.
       #
@@ -31,6 +32,7 @@ def operator
       def expression
         node_parts.last
       end
+      alias rhs expression
     end
   end
 end