add test for n*n=2n bits multiplication

[r15764]

ir/be/test/mul2n.c

+/* should produce only ONE mul, no conv's */
+#include <stdio.h>
+
+int a = 0x80000000;
+
+int main(void)
+{
+	long long x = (long long)a * a;
+	printf("%lld\n", x);
+	return 0;
+}