openmp: Improve composite triangular loop lowering and expansion

openmp: Improve composite triangular loop lowering and expansion

This propagates needed values from the point where number of iterations
is calculated on composite loops to the places where that information
is needed to use the more efficient square root discovery to compute
the starting iterator values from the logical iteration number.

2020-10-13  Jakub Jelinek  <jakub@redhat.com>

	* omp-low.c (add_taskreg_looptemp_clauses): For triangular loops
	with non-constant number of iterations add another 4 _looptemp_
	clauses before the (optional) one for lastprivate.
	(lower_omp_for_lastprivate): Skip those clauses when looking for
	the lastprivate clause.
	(lower_omp_for): For triangular loops with non-constant number of
	iterations add another 4 _looptemp_ clauses.
	* omp-expand.c (expand_omp_for_init_counts): For triangular loops
	with non-constant number of iterations set counts[0],
	fd->first_inner_iterations, fd->factor and fd->adjn1 from the newly
	added _looptemp_ clauses.
	(expand_omp_for_init_vars): Initialize the newly added _looptemp_
	clauses.
	(find_lastprivate_looptemp): New function.
	(expand_omp_for_static_nochunk, expand_omp_for_static_chunk,
	expand_omp_taskloop_for_outer): Use it instead of manually skipping
	_looptemp_ clauses.

(cherry picked from commit 14707c896a207606f13886d3b3251e8db1f3c9c0)