[patch] Rework RTL CFG graph dumping to dump DOT format

[Steven Bosscher]

Hello,

The graph.[ch] code to dump the CFG for viewing with VCG is quite
broken and hasn't been updated to follow the advances in the CFG
infrastructure.

The attached patch is the first in a series of patches I have planned
to update the CFG-as-graph dumping code to dump GraphViz DOT input
instead. The attached archive is an example input/output.

There's still quite a lot of work to be done:
- use pretty-print in the slim RTL dumping
- move the escaped-string printing into pretty-print
- make the basic block content dumping a cfghook
- make these dumps work for GIMPLE also

For the moment, this is a good starting point.

Obviously there are no changes in code generation due to this patch,
so IMHO this patch (and the rest of the series) should be safe for
stage3. Hopefully everyone else agrees :-)

Bootstrapped&tested on powerpc64-unknown-linux-gnu. OK for trunk?

Ciao!
Steven
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