vm_opt_ltlt: call rb_str_buf_append directly if RHS is a String

`rb_str_concat` does a lot of type checking we can easily bypass.

```

|               |compare-ruby|built-ruby|
|:--------------|-----------:|---------:|
|string_concat  |    362.007k|  398.965k|
|               |           -|     1.10x|
```

benchmark/string_concat.yml

@@ -0,0 +1,13 @@
+prelude: |
+  CHUNK = "a" * 64
+benchmark:
+  string_concat: |
+    buffer = String.new(capacity: 4096)
+    buffer << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK
+    buffer << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK
+    buffer << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK
+    buffer << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK
+    buffer << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK
+    buffer << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK
+    buffer << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK
+    buffer << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK

vm_insnhelper.c

@@ -5376,7 +5376,11 @@ vm_opt_ltlt(VALUE recv, VALUE obj)
     }
     else if (RBASIC_CLASS(recv) == rb_cString &&
 	     BASIC_OP_UNREDEFINED_P(BOP_LTLT, STRING_REDEFINED_OP_FLAG)) {
-	return rb_str_concat(recv, obj);
+	if (LIKELY(RB_TYPE_P(obj, T_STRING))) {
+	    return rb_str_buf_append(recv, obj);
+	} else {
+	    return rb_str_concat(recv, obj);
+	}
     }
     else if (RBASIC_CLASS(recv) == rb_cArray &&
 	     BASIC_OP_UNREDEFINED_P(BOP_LTLT, ARRAY_REDEFINED_OP_FLAG)) {