when there's no need to call CALLER_SETUP_ARG and CALLER_REMOVE_EMPTY_KW_SPLAT
(i.e. !rb_splat_or_kwargs_p(ci) && !calling->kw_splat).

Micro benchmark:
```
$ benchmark-driver -v --rbenv 'before;after' benchmark/vm_send_cfunc.yml --repeat-count=4
before: ruby 2.8.0dev (2020-04-13T23:45:05Z master b9d3ceee) [x86_64-linux]
after: ruby 2.8.0dev (2020-04-14T00:48:52Z no-splat-fastpath 418d363722) [x86_64-linux]
Calculating -------------------------------------
                         before       after
       vm_send_cfunc    69.585M     88.724M i/s -    100.000M times in 1.437097s 1.127096s

Comparison:
                    vm_send_cfunc
               after:  88723605.2 i/s
              before:  69584737.1 i/s - 1.28x  slower
```

Optcarrot:
```
$ benchmark-driver -v --rbenv 'before;after' benchmark.yml --repeat-count=12 --output=all
before: ruby 2.8.0dev (2020-04-13T23:45:05Z master b9d3ceee) [x86_64-linux]
after: ruby 2.8.0dev (2020-04-14T00:48:52Z no-splat-fastpath 418d363722) [x86_64-linux]
Calculating -------------------------------------
                                       before                 after
Optcarrot Lan_Master.nes    50.76119601545175     42.73858236484051 fps
                            50.76388649761503     51.04211379912850
                            50.80930672252514     51.39455790755538
                            50.90236000778749     51.75656936556145
                            51.01744746340430     51.86875277356489
                            51.06495279015112     51.88692482485558
                            51.07785337168974     51.93429603190578
                            51.20163525187862     51.95768145071314
                            51.34671771913112     52.45577266040274
                            51.35918340835583     52.53163888762858
                            51.46641337418146     52.62172484121034
                            51.50835463462257     52.85064021113239
```