Remove call to gcc_unreachable in range-op.cc

The Ada testcase happens to stumble on the call to gcc_unreachable in
operator_bitwise_xor::op1_range, but there is nothing wrong going on
and it's safe to let it go through.

gcc/
	* range-op.cc (get_bool_state): Adjust head comment.
	(operator_not_equal::op1_range): Fix comment.
	(operator_bitwise_xor::op1_range): Remove call to gcc_unreachable.
gcc/testsuite/
	* gnat.dg/specs/opt5.ads: New test.
	* gnat.dg/specs/opt5_pkg.ads: New helper.