[FEA] Avoid parsing field name twice when matching named instruction in get_json_object

[ttnghia]

Currently, in get_json_object kernel, each field name is parsed once to verify its validity, then it is parsed again to check if it is matched with the given named instruction. That is inefficient especially if we have long field names.

We can do better by optimizing this code path, parsing the field names only once. That means string validation and name matching can be executed together.