openmp: For min/max omp atomic compare forms verify arg types with build_binary_op...

openmp: For min/max omp atomic compare forms verify arg types with build_binary_op [PR104531]

The MIN_EXPR/MAX_EXPR handling in *build_binary_op is minimal (especially
for C FE), because min/max aren't expressions the languages contain directly.
I'm using those for the
  #pragma omp atomic
  x = x < y ? y : x;
forms, but e.g. for the attached testcase we normally reject _Complex int vs. int
comparisons, in C++ due to MIN/MAX_EXPR we were diagnosing it as invalid types
for <unknown> while in C we accept it and ICEd later on.

The following patch will try build_binary_op with LT_EXPR on the operands first
to get needed diagnostics and fail if it returns error_mark_node.

2022-02-16  Jakub Jelinek  <jakub@redhat.com>

	PR c/104531
	* c-omp.cc (c_finish_omp_atomic): For MIN_EXPR/MAX_EXPR, try first
	build_binary_op with LT_EXPR and only if that doesn't return
	error_mark_node call build_modify_expr.

	* c-c++-common/gomp/atomic-31.c: New test.