Add a new opcode that returns nil without requiring LOADNIL + RETURN.
This avoids loading nil into a register by setting the return value (v)
directly. The implementation uses a separate label (L_RETURN_NIL) to
bypass v = regs[a], preserving self in regs[0] for ensure blocks.
Codegen applies peephole optimization to fuse LOADNIL + RETURN -> RETNIL.
Co-authored-by: Claude <noreply@anthropic.com>
Yukihiro "Matz" Matsumoto
0b1af858e2