Fix thinko in operator_bitwise_xor::op1_range

There is a thinko in the op1_range method of ranger's operator_bitwise_xor
class in a boolean context: if the result is known to be true, it may infer
that a specific operand is false without any basis.

gcc/
	* range-op.cc (operator_bitwise_xor::op1_range): Fix thinko.

gcc/testsuite/
	* gnat.dg/opt100.adb: New test.
	* gnat.dg/opt100_pkg.adb, gnat.dg/opt100_pkg.ads: New helper.