[patch] fix .DOT file generation for IPA passes

[Aldy Hernandez <aldyh at redhat dot com>]

I was trying to generate a graph file with 
-fdump-ipa-tmipa-blocks-details-vops-graph, but the .dot file was 
corrupted.  It looks like the header bits printed in start_graph_dump() 
are not dumped because we are predicating the calls to 
clean_graph_dump_file->start_graph_dump by:

	  && cfun && (cfun->curr_properties & PROP_cfg))

The problem is that for IPA passes (well at least for tmipa) cfun is 
NULL so we don't initialize the dump file, but later we go through each 
function (setting cfun appropriately) and dump the corresponding graphs 
somewhere in:

    do_per_function (execute_function_dump, NULL);

I have fixed this by adding a bit in opt_pass to keep track of if a 
graph .DOT file has been initialized, and initialize it if not.  I 
suppose we could move initialization of the graph file further down, but 
that seemed a bit more tedious given all the places where we dump.

OK?

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