Add testcase for PR103615

This adds a testcase for a fixed wrong-code bug.

2022-01-03  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/103615
	* gcc.dg/torture/pr103615.c: New testcase.

gcc/testsuite/gcc.dg/torture/pr103615.c

+/* { dg-do run } */
+
+int z = 5;
+int a[6] = { 0, 0, 0, 0, 0, 1 };
+int main()
+{
+  for (int x = 5; x; x--)
+    for (int y = z; y >= x; y--)
+      a[y - x] += a[y];
+  if (a[0] != 7)
+    __builtin_abort ();
+  return 0;
+}