optimize method dispatch for lead/opt params.

There is a special optimization for "only lead parameters"
method dispatch using specialized dispatcher functions
`vm_call_iseq_setup_normal_0start...`.
Other cases (opt, rest, post, ...) we don't use specialized
dispatcher and call with `setup_parameters_complex` to
satisfy Ruby's complex parameter specification.

This commit introduce a specialize dispatcher for
methods which use only lead and optional parameters.

Two step improvements:
(1) prepare "lead/opt" only check pass.
    It is to skip the `setup_parameters_complex` function.
(2) introduce specialized dispatcher for only "lead/opt"
    parameters methods (vm_call_iseq_setup_normal_opt_start).

With these improvements, we achieved good micro-benchmark
results:
  With a method: `def opt2 a, b=nil; end`
  With the following binaries:
    clean-miniruby: unmodified trunk.
    opt_miniruby: apply step (1).
    opt_cc_miniruby: apply step (2).
  Result with benchmark-driver:

                        opt2(1)
   opt_cc_miniruby:  42269409.1 i/s
      opt_miniruby:  36304428.3 i/s - 1.16x  slower
    clean-miniruby:  25897409.5 i/s - 1.63x  slower

                     opt2(1, 2)
   opt_cc_miniruby:  45935145.7 i/s
      opt_miniruby:  40513196.9 i/s - 1.13x  slower
    clean-miniruby:  29976057.6 i/s - 1.53x  slower

This improvement may be trivial (difficult to improve practical
cases). However, this is enough small patch so I decide to
introduce it.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@67315 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

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