Simply use DISPATCH_ORIGINAL_INSN instead of rb_funcall.  This is,
when possible, overall performant because method dispatch results are
cached inside of CALL_CACHE.  Should also be good for JIT.

----

trunk: ruby 2.6.0dev (2018-09-12 trunk 64689) [x86_64-darwin15]
ours: ruby 2.6.0dev (2018-09-12 leaf-insn 64688) [x86_64-darwin15]
last_commit=make opt_str_freeze leaf
Calculating -------------------------------------
                          trunk        ours
    vm2_freezestring     5.440M     31.411M i/s -      6.000M times in 1.102968s 0.191017s

Comparison:
                 vm2_freezestring
                ours:  31410864.5 i/s
               trunk:   5439865.4 i/s - 5.77x  slower



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@64690 b2dd03c8-39d4-4d8f-98ff-823fe69b080e