added test from lin2vareq to affeq that found mistake in dim_add

tests/regression/63-affeq/21-function_call.c

@@ -0,0 +1,21 @@
+// SKIP PARAM: --set ana.activated[+] affeq --set sem.int.signed_overflow assume_none
+// This test was added from 77-lin2vareq because it found mistakes in dim_add that weren't detected by the other tests
+#include <goblint.h>
+
+int check_equal(int x, int y, int z) {
+  __goblint_check(x == y);
+  __goblint_check(z == y);
+  __goblint_check(x == z);
+  return 8;
+}
+
+int main(void) {
+  int x, y, z;
+
+  y = x;
+  z = y;
+
+  check_equal(x, y, z);
+
+  return 0;
+}