HFIR miscompile: encode_json(parse_json s) prints raw s when followed by a let argument #155

Found in #154. With (print (encode_json (parse_json Any s)) (let …)), HFIR bytecode prints the raw text of s instead of the re-encoded JSON. The interpreter and -run-bc are correct. This is also on main. Fix: correct the lowering, and add a three-path parity test for this shape.