Prevent a warning: assigned but unused variable

... which made a test fail

http://rubyci.s3.amazonaws.com/debian10/ruby-master/log/20230313T213003Z.fail.html.gz

```
  1) Failure:
TestRubyOptions#test_enable [/home/chkbuild/chkbuild/tmp/build/20230313T213003Z/ruby/test/ruby/test_rubyoptions.rb:176]:
pid 1208 exit 0
| /home/chkbuild/chkbuild/tmp/build/20230313T213003Z/ruby/lib/ruby_vm/rjit/insn_compiler.rb:1257: warning: assigned but unused variable - argc
| /home/chkbuild/chkbuild/tmp/build/20230313T213003Z/ruby/lib/ruby_vm/rjit/insn_compiler.rb:1258: warning: assigned but unused variable - flags
.

1. [2/2] Assertion for "stderr"
   | <[]> expected but was
   | <["/home/chkbuild/chkbuild/tmp/build/20230313T213003Z/ruby/lib/ruby_vm/rjit/insn_compiler.rb:1257: warning: assigned but unused variable - argc",
   |  "/home/chkbuild/chkbuild/tmp/build/20230313T213003Z/ruby/lib/ruby_vm/rjit/insn_compiler.rb:1258: warning: assigned but unused variable - flags"]>.
```

lib/ruby_vm/rjit/insn_compiler.rb

@@ -1254,8 +1254,8 @@ def invokeblock(jit, ctx, asm)
       # Get call info
       cd = C.rb_call_data.new(jit.operand(0))
       ci = cd.ci
-      argc = C.vm_ci_argc(ci)
-      flags = C.vm_ci_flag(ci)
+      _argc = C.vm_ci_argc(ci)
+      _flags = C.vm_ci_flag(ci)
 
       # Get block_handler
       cfp = jit.cfp