edge case for 64bit add where add with flag output is CSEd

[r15757]

ir/be/test/add64.c

+
+long long f(long long a, long long b)
+{
+	long long c = (a & 0xffffffff) | 0x100000000;
+	return a + b + c;
+}
+
+int main(int argc, char **argv) {
+	printf("Res: %llx\n", f(0x100000000, 0x100000000));
+}