insns.def: unwrap vm_exec for yield

Outer vm_exec can catch longjmp. We don't need to call vm_exec first
here.

This optimizes JIT-ed yield:

* Benchmark script

```
require 'benchmark_driver'

Benchmark.driver do |x|
  x.prelude %{
    def yielder
      yield + 1
    end
  }
  x.report 'yielder', %{
    yielder { 1 }
  }
  x.loop_count 300_000_000

  x.rbenv 'before', 'before,--jit', 'after', 'after,--jit'
  x.verbose
end
```

* Result

before: ruby 2.6.0dev (2018-03-03 trunk 62642) [x86_64-linux]
before,--jit: ruby 2.6.0dev (2018-03-03 trunk 62642) +JIT [x86_64-linux]
after: ruby 2.6.0dev (2018-03-03 trunk 62642) [x86_64-linux]
last_commit=insns.def: unwrap vm_exec for yield
after,--jit: ruby 2.6.0dev (2018-03-03 trunk 62642) +JIT [x86_64-linux]
last_commit=insns.def: unwrap vm_exec for yield
Calculating -------------------------------------
                         before  before,--jit       after  after,--jit
             yielder    37.214M       29.222M     35.904M      38.035M i/s -    300.000M times in 8.061581s 10.266312s 8.355716s 7.887447s

Comparison:
                          yielder
         after,--jit:  38035121.0 i/s
              before:  37213544.0 i/s - 1.02x  slower
               after:  35903565.7 i/s - 1.06x  slower
        before,--jit:  29221787.6 i/s - 1.30x  slower

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@62643 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

insns.def

@@ -819,8 +819,8 @@ invokeblock
     }
 
     val = vm_invoke_block(ec, GET_CFP(), &calling, ci, block_handler);
-    if (val == Qundef) {
-        EXEC_EC_CFP(TRUE);
+    if (val == Qundef && (val = mjit_exec(ec)) == Qundef) {
+        EXEC_EC_CFP(FALSE);
     }
 }