gdb: address review comments of previous series

I forgot to include fixes for review comments I got before pushing the
previous commits (or I pushed the wrong commits).  This one fixes it.

 - Return {} instead of false in get_discrete_low_bound and
   get_discrete_high_bound.
 - Compute high bound after confirming low bound is valid in
   get_discrete_bounds.

gdb/ChangeLog:

	* gdbtypes.c (get_discrete_low_bound, get_discrete_high_bound):
	Return {} instead of false.
	(get_discrete_bounds): Compute high bound only if low bound is
	valid.

Change-Id: I5f9a66b3672adfac9441068c899ab113ab2c331a

gdb/gdbtypes.c

@@ -1093,7 +1093,7 @@ get_discrete_low_bound (struct type *type)
 
     case TYPE_CODE_INT:
       if (TYPE_LENGTH (type) > sizeof (LONGEST))	/* Too big */
-	return false;
+	return {};
 
       if (!type->is_unsigned ())
 	return -(1 << (TYPE_LENGTH (type) * TARGET_CHAR_BIT - 1));
@@ -1103,7 +1103,7 @@ get_discrete_low_bound (struct type *type)
       return 0;
 
     default:
-      return false;
+      return {};
     }
 }
 
@@ -1160,7 +1160,7 @@ get_discrete_high_bound (struct type *type)
 
     case TYPE_CODE_INT:
       if (TYPE_LENGTH (type) > sizeof (LONGEST))	/* Too big */
-	return false;
+	return {};
 
       if (!type->is_unsigned ())
 	{
@@ -1179,7 +1179,7 @@ get_discrete_high_bound (struct type *type)
       }
 
     default:
-      return false;
+      return {};
     }
 }
 
@@ -1189,9 +1189,11 @@ bool
 get_discrete_bounds (struct type *type, LONGEST *lowp, LONGEST *highp)
 {
   gdb::optional<LONGEST> low = get_discrete_low_bound (type);
-  gdb::optional<LONGEST> high = get_discrete_high_bound (type);
+  if (!low.has_value ())
+    return false;
 
-  if (!low.has_value () || !high.has_value ())
+  gdb::optional<LONGEST> high = get_discrete_high_bound (type);
+  if (!high.has_value ())
     return false;
 
   *lowp = *low;