Fix UB in int_const_binop

When testing PR 85164, the baseline bootstrap-ubsan results had
a lot of failures from int_const_binop.  This is because with the
new overflow handling we can sometimes do:

      poly_res = res;

on an uninitialised res.

2019-04-18  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* fold-const.c (int_const_binop): Return early on failure.

From-SVN: r270443

gcc/ChangeLog

 2019-04-18  Richard Sandiford  <richard.sandiford@arm.com>
 
+	* fold-const.c (int_const_binop): Return early on failure.
+
+2019-04-18  Richard Sandiford  <richard.sandiford@arm.com>
+
 	PR middle-end/85164
 	* combine.c (force_int_to_mode): Cast the argument rather than
 	the result of known_alignment.

gcc/fold-const.c

@@ -1173,7 +1173,6 @@ tree
 int_const_binop (enum tree_code code, const_tree arg1, const_tree arg2,
 		 int overflowable)
 {
-  bool success = false;
   poly_wide_int poly_res;
   tree type = TREE_TYPE (arg1);
   signop sign = TYPE_SIGN (type);
@@ -1183,17 +1182,18 @@ int_const_binop (enum tree_code code, const_tree arg1, const_tree arg2,
     {
       wide_int warg1 = wi::to_wide (arg1), res;
       wide_int warg2 = wi::to_wide (arg2, TYPE_PRECISION (type));
-      success = wide_int_binop (res, code, warg1, warg2, sign, &overflow);
+      if (!wide_int_binop (res, code, warg1, warg2, sign, &overflow))
+	return NULL_TREE;
       poly_res = res;
     }
-  else if (poly_int_tree_p (arg1) && poly_int_tree_p (arg2))
-    success = poly_int_binop (poly_res, code, arg1, arg2, sign, &overflow);
-  if (success)
+  else if (!poly_int_tree_p (arg1)
+	   || !poly_int_tree_p (arg2)
+	   || !poly_int_binop (poly_res, code, arg1, arg2, sign, &overflow))
+    return NULL_TREE;
   return force_fit_type (type, poly_res, overflowable,
 			 (((sign == SIGNED || overflowable == -1)
 			   && overflow)
 			  | TREE_OVERFLOW (arg1) | TREE_OVERFLOW (arg2)));
-  return NULL_TREE;
 }
 
 /* Return true if binary operation OP distributes over addition in operand