Dump before flag

[Xinliang David Li]

Hi, this is a simple patch that support dump_before flag. E.g,

-fdump-tree-pre-before

This is useful for diffing the the IR before and after a pass.

Gcc dumping needs more cleanups -- such as allowing IR only dump,
allowing IR dumping for a particular function etc. The exposure of
'dumpfile' (instead of a dumping_level () function) makes those change
a little messy, but can be done.

Ok for trunk?

Thanks,

David