openmp, fortran: Improve !$omp atomic checks [PR104328]

openmp, fortran: Improve !$omp atomic checks [PR104328]

The testcase shows some cases that weren't verified and we ICE on
invalid because of that.
One problem is that unlike before, we weren't checking if some expression
is EXPR_VARIABLE with non-NULL symtree in the case where there was
a conversion around it.
The other two issues is that we check that in an IF ->block is non-NULL
and then immediately dereference ->block->next->op, but on invalid
code with no statements in the then clause ->block->next might be NULL.

2022-02-02  Jakub Jelinek  <jakub@redhat.com>

	PR fortran/104328
	* openmp.cc (is_scalar_intrinsic_expr): If must_be_var && conv_ok
	and expr is conversion, verify it is a conversion from EXPR_VARIABLE
	with non-NULL symtree.  Check ->block->next before dereferencing it.

	* gfortran.dg/gomp/atomic-27.f90: New test.