pass_manager: Fix ICE with -fdump-passes -fdisable-tree-* [PR93874]

pass_manager: Fix ICE with -fdump-passes -fdisable-tree-* [PR93874]

dump_passes pushes a dummy function for which it evaluates the gates
and checks whether the pass is enabled or disabled.
Unfortunately, if any -fdisable-*-*/-fenable-*-* options were seen,
we ICE during is_pass_explicitly_enabled_or_disabled because slot
is non-NULL then and the code will do:
  cgraph_uid = func ? cgraph_node::get (func)->get_uid () : 0;
but the dummy function doesn't have a cgraph node.

So, either we need to create and then remove a cgraph node for the dummy
function like the following patch, or function.c would need to export the
in_dummy_function flag (or have some way to query that flag from other TUs)
and we'd need to check it in is_pass_explicitly_enabled_or_disabled.

2020-02-25  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/93874
	* passes.c (pass_manager::dump_passes): Create a cgraph node for the
	dummy function and remove it at the end.

	* gcc.dg/pr93874.c: New test.