OpenMP/Fortran: Partially fix non-rect loop nests [PR107424]

This patch ensures that loop bounds depending on outer loop vars use the
proper TREE_VEC format. It additionally gives a sorry if such an outer
var has a non-one/non-minus-one increment as currently a count variable
is used in this case (see PR).

Finally, it avoids 'count' and just uses a local loop variable if the
step increment is +/-1.

	PR fortran/107424

gcc/fortran/ChangeLog:

	* trans-openmp.cc (struct dovar_init_d): Add 'sym' and
	'non_unit_incr' members.
	(gfc_nonrect_loop_expr): New.
	(gfc_trans_omp_do): Call it; use normal loop bounds
	for unit stride - and only create local loop var.

libgomp/ChangeLog:

	* testsuite/libgomp.fortran/non-rectangular-loop-1.f90: New test.
	* testsuite/libgomp.fortran/non-rectangular-loop-1a.f90: New test.
	* testsuite/libgomp.fortran/non-rectangular-loop-2.f90: New test.
	* testsuite/libgomp.fortran/non-rectangular-loop-3.f90: New test.
	* testsuite/libgomp.fortran/non-rectangular-loop-4.f90: New test.
	* testsuite/libgomp.fortran/non-rectangular-loop-5.f90: New test.

gcc/testsuite/ChangeLog:

	* gfortran.dg/goacc/privatization-1-compute-loop.f90: Update dg-note.
	* gfortran.dg/goacc/privatization-1-routine_gang-loop.f90: Likewise.