[range-ops] Handle bitmasks for unary operators.

It looks like we missed out on bitmasks for unary operators because we
were using bit_value_binop exclusively.  This patch hands off to
bit_value_unop when appropriate, thus allowing us to handle ABS and
BIT_NOT_EXPR, and others.  Follow-up patches will add the tweaks for the
range-ops entries themselves.

gcc/ChangeLog:

	* range-op.cc (update_known_bitmask): Handle unary operators.