Adjust testcase for O2 vectorization enabling

This issue was observed in rs6000 specific PR102658 as well.

I've looked into it a bit, it's caused by the "conditional store replacement" which
is originally disabled without vectorization as below code.

  /* If either vectorization or if-conversion is disabled then do
     not sink any stores.  */
  if (param_max_stores_to_sink == 0
      || (!flag_tree_loop_vectorize && !flag_tree_slp_vectorize)
      || !flag_tree_loop_if_convert)
    return false;

The new change makes the innermost loop look like

for (int c1 = 0; c1 <= 1499; c1 += 1) {
  if (c1 <= 500) {
     S_10(c0, c1);
  } else {
      S_9(c0, c1);
  }
  S_11(c0, c1);
}

and can not be splitted as:

for (int c1 = 0; c1 <= 500; c1 += 1)
  S_10(c0, c1);

for (int c1 = 501; c1 <= 1499; c1 += 1)
  S_9(c0, c1);

So instead of disabling vectorization, could we just disable this cs replacement
with parameter "--param max-stores-to-sink=0"?

I tested this proposal on ppc64le, it should work as well.

2021-10-11  Kewen Lin  <linkw@linux.ibm.com>

libgomp/ChangeLog:

	* testsuite/libgomp.graphite/force-parallel-8.c: Add --param max-stores-to-sink=0.