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.

gcc/range-op.cc

@@ -358,10 +358,8 @@ range_true_and_false (tree type)
 
 enum bool_range_state { BRS_FALSE, BRS_TRUE, BRS_EMPTY, BRS_FULL };
 
-// Return the summary information about boolean range LHS.  Return an
-// "interesting" range in R.  For EMPTY or FULL, return the equivalent
-// range for TYPE, for BRS_TRUE and BRS false, return the negation of
-// the bool range.
+// Return the summary information about boolean range LHS.  If EMPTY/FULL,
+// return the equivalent range for TYPE in R; if FALSE/TRUE, do nothing.
 
 static bool_range_state
 get_bool_state (irange &r, const irange &lhs, tree val_type)
@@ -383,6 +381,7 @@ get_bool_state (irange &r, const irange &lhs, tree val_type)
       r.set_varying (val_type);
       return BRS_FULL;
     }
+
   return BRS_TRUE;
 }
 
@@ -538,7 +537,7 @@ operator_not_equal::op1_range (irange &r, tree type,
       break;
 
     case BRS_FALSE:
-      // If its true, the result is the same as OP2.
+      // If it's false, the result is the same as OP2.
       r = op2;
       break;
 
@@ -2643,7 +2642,7 @@ operator_bitwise_xor::op1_range (irange &r, tree type,
 	  r = op2;
 	  break;
 	default:
-	  gcc_unreachable ();
+	  break;
 	}
       return true;
     }

gcc/testsuite/gnat.dg/specs/opt5.ads

@@ -0,0 +1,25 @@
+-- { dg-do compile }
+-- { dg-options "-O2 -gnata -gnatVa" }
+
+with Opt5_Pkg;
+
+package Opt5 is
+
+  type Object is new Opt5_Pkg.Object with private;
+
+  Undefined : constant Object;
+
+  overriding function Is_Defined (Self : Object) return Boolean;
+
+  function Create (Sloc : Opt5_Pkg.Object) return Integer is (0)
+    with Pre  => Sloc.Is_Defined;
+
+private
+
+  type Object is new Opt5_Pkg.Object with null record;
+
+  Undefined : constant Object := (Opt5_Pkg.Undefined with others => <>);
+
+  overriding function Is_Defined (Self : Object) return Boolean is (Self /= Undefined);
+
+end Opt5;

gcc/testsuite/gnat.dg/specs/opt5_pkg.ads

@@ -0,0 +1,17 @@
+package Opt5_Pkg is
+
+  type Object is tagged private;
+
+  Undefined : constant Object;
+
+  function Is_Defined (Self : Object) return Boolean;
+
+private
+
+  type Object is tagged null record;
+
+  Undefined : constant Object := (others => <>);
+
+  function Is_Defined (Self : Object) return Boolean is (Self /= Undefined);
+
+end Opt5_Pkg;