Utility to dump .dot representation of SelectionDAG without firing viewer

Summary:
This patch adds support for dumping .dot
representation of SelectionDAG. It is inspired from the fact that,
a developer may want to just dump the graph at
a predictable path with a simple name to compare.
The exisitng utility (i.e. viewGraph) are overkill
for this motive hence this patch adds the requires support
while using the core routines from GraphWriter.

Example usage: DAG.dumpDotGraph("/tmp/graph.dot", "MyGraph")
will create /tmp/graph.dot file when DAG is an
object of SelectionDAG class.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D80711