call.c (null_ptr_cst_p): Replace use of TREE_CONSTANT_OVERFLOW with TREE_OVERFLOW.

	* call.c (null_ptr_cst_p): Replace use of TREE_CONSTANT_OVERFLOW with
	TREE_OVERFLOW.
	* typeck.c (ignore_overflows): Remove the remaining uses of
	TREE_CONSTANT_OVERFLOW.

From-SVN: r121084