omp-expand.cc: Fix wrong code with non-rectangular loop nest [PR111017]

Before commit r12-5295-g47de0b56ee455e, all gimple_build_cond in
expand_omp_for_* were inserted with
  gsi_insert_before (gsi_p, cond_stmt, GSI_SAME_STMT);
except the one dealing with the multiplicative factor that was
  gsi_insert_after (gsi, cond_stmt, GSI_CONTINUE_LINKING);

That commit for PR103208 fixed the issue of some missing regimplify of
operands of GIMPLE_CONDs by moving the condition handling to the new function
expand_omp_build_cond. While that function has an 'bool after = false'
argument to switch between the two variants.

However, all callers ommited this argument. This commit reinstates the
prior behavior by passing 'true' for the factor != 0 condition, fixing
the included testcase.

	PR middle-end/111017
gcc/
	* omp-expand.cc (expand_omp_for_init_vars): Pass after=true
	to expand_omp_build_cond for 'factor != 0' condition, resulting
	in pre-r12-5295-g47de0b56ee455e code for the gimple insert.

libgomp/
	* testsuite/libgomp.c-c++-common/non-rect-loop-1.c: New test.

(cherry picked from commit 1dc65003)