MFC r365628 by ae: Fix compatibility regression after r364117.

Properly handle the case, when some opcode keywords follow after
the frag opcode without additional options.

Reported by: Evgeniy Khramtsov <evgeniy at khramtsov org>
Approved by: glebius

(cherry picked from commit b876085f5589f4e6d531d97ad902c5730ec55339)