openmp: Mark OpenMP atomic write expression as read [PR119000]

The following testcase was emitting false positive warning that
the rhs of #pragma omp atomic write was stored but not read,
when the atomic actually does read it.  The following patch
fixes that by calling default_function_array_read_conversion
on it, so that it is marked as read as well as converted from
lvalue to rvalue.
Furthermore, the code had
if (code == NOP_EXPR) ... else ... if (code == NOP_EXPR) ...
with none of ... parts changing code, so I've merged the two ifs.

2025-02-25  Jakub Jelinek  <jakub@redhat.com>

	PR c/119000
	* c-parser.cc (c_parser_omp_atomic): For omp write call
	default_function_array_read_conversion on the rhs expression.
	Merge the two adjacent if (code == NOP_EXPR) blocks.

	* c-c++-common/gomp/pr119000.c: New test.