Fix an error for Performance/RegexpMatch

Follow up rubocop/rubocop-performance#73 (comment).

This commit fixes an error for `Performance/RegexpMatch` when
a regexp used independently with a regexp used in `if` are mixed.

lib/rubocop/cop/performance/regexp_match.rb

@@ -183,27 +183,21 @@ def last_match_used?(match_node)
         end
 
         def range_to_search_for_last_matches(match_node, body, scope_root)
-          expression = if (modifier_form = modifier_form?(match_node))
+          expression = if modifier_form?(match_node)
                          match_node.parent.if_branch.loc.expression
                        else
                          match_node.loc.expression
                        end
 
           match_node_pos = expression.begin_pos
-          next_match_pos = next_match_pos(
-            body, match_node_pos, scope_root, modifier_form
-          )
+          next_match_pos = next_match_pos(body, match_node_pos, scope_root)
 
           match_node_pos..next_match_pos
         end
 
-        def modifier_form?(match_node)
-          match_node.parent.if_type? && match_node.parent.modifier_form?
-        end
-
-        def next_match_pos(body, match_node_pos, scope_root, modifier_form)
+        def next_match_pos(body, match_node_pos, scope_root)
           node = search_match_nodes(body).find do |match|
-            begin_pos = if modifier_form
+            begin_pos = if modifier_form?(match)
                           match.parent.if_branch.loc.expression.begin_pos
                         else
                           match.loc.expression.begin_pos
@@ -215,6 +209,10 @@ def next_match_pos(body, match_node_pos, scope_root, modifier_form)
           node ? node.loc.expression.begin_pos : Float::INFINITY
         end
 
+        def modifier_form?(match_node)
+          match_node.parent.if_type? && match_node.parent.modifier_form?
+        end
+
         def find_last_match(body, range, scope_root)
           last_matches(body).find do |ref|
             ref_pos = ref.loc.expression.begin_pos

spec/rubocop/cop/performance/regexp_match_spec.rb

@@ -432,5 +432,17 @@ def foo
         end
       RUBY
     end
+
+    it 'registers an offense when a regexp used independently ' \
+       'with a regexp used in `if` are mixed' do
+      expect_offense(<<-RUBY)
+        def foo
+          /re/ === re # A regexp used independently is not yet supported.
+
+          do_something if /re/ === re
+                          ^^^^^^^^^^^ Use `match?` instead of `===` when `MatchData` is not used.
+        end
+      RUBY
+    end
   end
 end