[OpenACC] Fix an ICE where a loop with GT condition is collapsed.

[OpenACC] Fix an ICE where a loop with GT condition is collapsed.

We have seen an ICE both on trunk and devel/omp/gcc-10 branches which can
be reprodued with this simple testcase.  It occurs if an OpenACC loop has
a collapse clause and any of the loop being collapsed uses GT or GE
condition.  This issue is specific to OpenACC.

int main (void)
{
  int ix, iy;
  int dim_x = 16, dim_y = 16;
  {
       for (iy = dim_y - 1; iy > 0; --iy)
       for (ix = dim_x - 1; ix > 0; --ix)
        ;
  }
}

The problem is caused by a failing assertion in expand_oacc_collapse_init.
It checks that cond_code for fd->loop should be same as cond_code for all
the loops that are being collapsed.  As the cond_code for fd->loop is
LT_EXPR with collapse clause (set at the end of omp_extract_for_data),
this assertion forces that all the loop in collapse clause should use
< operator.

There does not seem to be anything in the code which demands this
condition as loop with > condition works ok otherwise.  I digged old
mailing list a bit but could not find any discussion on this change.
Looking at the code, expand_oacc_for checks that fd->loop->cond_code is
either LT_EXPR or GT_EXPR.  I guess the original intention was to have
similar checks on the loop which are being collapsed. But the way check
was written does not acheive that.

I have fixed it by modifying the check in the assertion to be same as
check on fd->loop->cond_code.

I tested goacc and libgomp (with nvptx offloading) and did not see any
regression.  I have added new tests to check collapse with GT/GE condition.

	PR middle-end/98088
	gcc/
	* omp-expand.c (expand_oacc_collapse_init): Update condition in
	a gcc_assert.

	gcc/testsuite/
	* c-c++-common/goacc/collapse-2.c: New.

	libgomp/
	* testsuite/libgomp.oacc-c-c++-common/collapse-2.c: Add check
	for loop with GT/GE condition.
	* testsuite/libgomp.oacc-c-c++-common/collapse-3.c: Likewise.