Simplify and optimize.

This reduces memory allocation from 19MB to 1.9MB (when run on cop/mixin).
Main issue remains that it is called way too many times

lib/rubocop/ast/node_pattern.rb

@@ -133,6 +133,8 @@ class Compiler
         PARAM_NUMBER = /%\d*/.freeze
 
         SEPARATORS = /\s+/.freeze
+        ONLY_SEPARATOR = /\A#{SEPARATORS}\Z/.freeze
+
         TOKENS     = Regexp.union(META, PARAM_CONST, KEYWORD_NAME, PARAM_NUMBER, NUMBER,
                                   METHOD_NAME, SYMBOL, STRING)
 
@@ -783,7 +785,7 @@ def substitute_cur_node(code, cur_node, first_cur_node: cur_node)
         end
 
         def self.tokens(pattern)
-          pattern.scan(TOKEN).reject { |token| token =~ /\A#{SEPARATORS}\Z/ }
+          pattern.scan(TOKEN).grep_v(ONLY_SEPARATOR)
         end
 
         def def_helper(base, method_name, **defaults)