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.

gcc/range-op.cc

@@ -3529,6 +3529,9 @@ operator_bitwise_xor::op1_range (irange &r, tree type,
 	    r.set_varying (type);
 	  else if (op2.zero_p ())
 	    r = range_true (type);
+	  // See get_bool_state for the rationale
+	  else if (op2.contains_p (build_zero_cst (op2.type ())))
+	    r = range_true_and_false (type);
 	  else
 	    r = range_false (type);
 	  break;

gcc/testsuite/gnat.dg/opt100.adb

@@ -0,0 +1,13 @@
+-- { dg-do run }
+-- { dg-options "-O2 -gnatp" }
+
+with Opt100_Pkg; use Opt100_Pkg;
+
+procedure Opt100 is
+  R : constant Rec := (K => B, N => 1);
+
+begin
+  if Func (R) /= 1 then
+     raise Program_Error;
+  end if;
+end;

gcc/testsuite/gnat.dg/opt100_pkg.adb

@@ -0,0 +1,17 @@
+package body Opt100_Pkg is
+
+   function Func (R : Rec) return Integer is
+   begin
+      if R in Small_Rec then
+         case R.K is
+            when A => return 0;
+            when B => return 1;
+            when C => return 2;
+            when others => raise Program_Error;
+         end case;
+      else
+         return -1;
+      end if;
+   end;
+
+end Opt100_Pkg;

gcc/testsuite/gnat.dg/opt100_pkg.ads

@@ -0,0 +1,23 @@
+with Interfaces; use Interfaces;
+
+package Opt100_Pkg is
+
+  A : constant Unsigned_8 := 0;
+  B : constant Unsigned_8 := 1;
+  C : constant Unsigned_8 := 2;
+
+  subtype Small_Unsigned_8 is Unsigned_8 range A .. C;
+
+  type Rec is record
+    K : Unsigned_8;
+    N : Natural;
+  end record;
+
+  subtype Small_Rec is Rec
+    with Dynamic_Predicate =>
+      Small_Rec.K in Small_Unsigned_8 and
+        ((Small_Rec.N in Positive) = (Small_Rec.K in B | C));
+
+   function Func (R : Rec) return Integer;
+
+end Opt100_Pkg;