openmp: Fix ICE on non-rectangular loop with known 0 iterations

openmp: Fix ICE on non-rectangular loop with known 0 iterations

The loops in the testcase are non-rectangular and have 0 iterations
(the outer loop iterates, but the inner one never).  In this case we
just have the overall number of iterations computed (0), and don't have
factor and other values computed.  We never need to map logical iterations
to the individual iterations in that case, and we were crashing during
expansion of that code.

2020-11-18  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/97862
	* omp-expand.c (expand_omp_for_init_vars): Don't use the sqrt path
	if number of iterations is constant 0.

	* c-c++-common/gomp/pr97862.c: New test.

(cherry picked from commit ba009860aec4619f2424f5bdee812f14572dc3cc)