OP_SASSIGN: make op_first==op_last for UNOP

OP_SASSIGN: make op_first==op_last for UNOP

Occasionally (e.g. $x ||= 1) an OP_SASSIGN operator only has a single
arg. The previous two commits made OP_SASSIGN always be allocated as a
BINOP, and if necessary, set op_last to NULL when there's only s single
arg.

This commit instead sets op_last equal to op_first for this case (similar
to how a LISTOP with a single arg is handled). This removes the need for
special handling in S_finalize_op().