Fix a false positive for Performance/RegexpMatch

Follow up rubocop/rubocop#5569 (comment).

This commit fixes a false positive for `Performance/RegexpMatch`
when `MatchData` exists after guard condition.

The following is an example code with false positive.

```ruby
def foo
  return if /re/ =~ str

  # `Regexp.last_match[1]` cannot be referenced if changed from `=~` to `match?`.
  do_something(Regexp.last_match[1])
end
```

.rubocop_todo.yml

@@ -1,6 +1,6 @@
 # This configuration was generated by
 # `rubocop --auto-gen-config`
-# on 2019-07-27 17:51:55 +0800 using RuboCop version 0.73.0.
+# on 2019-07-27 22:39:00 +0800 using RuboCop version 0.73.0.
 # The point is for the user to remove these configuration records
 # one by one as the offenses are removed from the code base.
 # Note that changes in the inspected code, or installation of new
@@ -22,9 +22,9 @@ Metrics/AbcSize:
 # Offense count: 2
 # Configuration parameters: CountComments.
 Metrics/ClassLength:
-  Max: 156
+  Max: 164
 
-# Offense count: 15
+# Offense count: 14
 # Configuration parameters: CountComments, ExcludedMethods.
 Metrics/MethodLength:
   Max: 14
@@ -44,7 +44,7 @@ RSpec/ContextWording:
     - 'spec/rubocop/cop/performance/string_replacement_spec.rb'
     - 'spec/rubocop/cop/performance/times_map_spec.rb'
 
-# Offense count: 88
+# Offense count: 89
 # Configuration parameters: Max.
 RSpec/ExampleLength:
   Enabled: false

lib/rubocop/cop/performance/regexp_match.rb

@@ -177,30 +177,41 @@ def last_match_used?(match_node)
           scope_root = scope_root(match_node)
           body = scope_root ? scope_body(scope_root) : match_node.ancestors.last
 
-          range =
-            if match_node.parent.if_type? && match_node.parent.modifier_form?
-              if_branch_range(match_node)
-            else
-              match_node_pos = match_node.loc.expression.begin_pos
-              next_match_pos = next_match_pos(body, match_node_pos, scope_root)
-
-              match_node_pos..next_match_pos
-            end
+          range = range_to_search_for_last_matches(match_node, body, scope_root)
 
           find_last_match(body, range, scope_root)
         end
 
-        def if_branch_range(match_node)
-          expression = match_node.parent.if_branch.loc.expression
+        def range_to_search_for_last_matches(match_node, body, scope_root)
+          expression = if (modifier_form = 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
+          )
+
+          match_node_pos..next_match_pos
+        end
 
-          expression.begin_pos..expression.end_pos
+        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)
+        def next_match_pos(body, match_node_pos, scope_root, modifier_form)
           node = search_match_nodes(body).find do |match|
-            match.loc.expression.begin_pos > match_node_pos &&
-              scope_root(match) == scope_root
+            begin_pos = if modifier_form
+                          match.parent.if_branch.loc.expression.begin_pos
+                        else
+                          match.loc.expression.begin_pos
+                        end
+
+            begin_pos > match_node_pos && scope_root(match) == scope_root
           end
+
           node ? node.loc.expression.begin_pos : Float::INFINITY
         end
 

spec/rubocop/cop/performance/regexp_match_spec.rb

@@ -138,6 +138,37 @@ def foo
         RUBY
       end
 
+      it "accepts #{name} in guard condition with " \
+         "#{var} is used in the line after that" do
+        expect_no_offenses(<<-RUBY)
+          def foo
+            return if #{cond}
+
+            do_something(#{var})
+          end
+        RUBY
+      end
+
+      include_examples 'offense',
+                       "#{name} in if guard condition with " \
+                       "#{var} is used in another method", <<-RUBY, <<-RUBY2
+        def foo
+          return if #{cond}
+        end
+
+        def bar
+          do_something(#{var})
+        end
+      RUBY
+        def foo
+          return if #{correction}
+        end
+
+        def bar
+          do_something(#{var})
+        end
+      RUBY2
+
       it "accepts #{name} in method with #{var} in block" do
         expect_no_offenses(<<-RUBY)
           def foo