optimize named capture assignment

* compile.c (compile_named_capture_assign): optimize named capture
  assignments, by replacing repeating global variable accesses
  with `dup`, and by returning the matched result instead of
  re-getting it from the MatchData.

* parse.y (reg_named_capture_assign_gen): build just assignment
  nodes for the optimization.

ex. `/(?<x>.)/ =~ "bar"`

- old
  ```
  0000 putstring        "bar"
  0002 opt_regexpmatch1 /(?<x>.)/
  0004 pop
  0005 getglobal        $~
  0007 branchunless     25
  0009 getglobal        $~
  0011 putobject        :x
  0013 opt_aref         <callinfo!mid:[], argc:1, ARGS_SIMPLE>
  0016 setlocal_OP__WC__0 2
  0018 getglobal        $~
  0020 putobject_OP_INT2FIX_O_0_C_
  0021 opt_send_without_block <callinfo!mid:begin, argc:1, ARGS_SIMPLE>
  0024 leave
  0025 putobject        nil
  0027 setlocal_OP__WC__0 2
  0029 putobject        nil
  0031 leave
  ```

- new
  ```
  0000 putstring        "bar"
  0002 opt_regexpmatch1 /(?<x>.)/
  0004 getglobal        $~
  0006 dup
  0007 branchunless     14
  0009 putobject        :x
  0011 opt_aref         <callinfo!mid:[], argc:1, ARGS_SIMPLE>
  0014 setlocal_OP__WC__0 2
  0016 leave
  ```

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@54100 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

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