[NFC] Generate .dot graph with monospace font for blocks, making it easier to read

[grosser]

This is a pretty subjective choice, I suppose. However, as must editors use monospace and that's where LLVM-IR is looked at most of the time, this likely makes sense. From my perspective this is fine. I leave some time for others to see if they have a differing opinion.

[sunfish]

My only concern would be if this significantly changes the overall size of nodes. Really big nodes sometimes make edges harder to read.

[grosser]

The size is relative (you can always zoom).

The shapes get relatively a little wider and less high.

In terms of readability, it seems to me that at the same size of text the monospace font is better readable (as a programming language). This is most visible when zooming out and font becomes very small. Again, this might be a subjective feeling.

From my feeling monospace font improves the readability a bit.

include/llvm/Support/GraphWriter.h

public:
   bool isNodeHidden(NodeRef Node) {
     return DTraits.isNodeHidden(Node);
   }
 
   void writeNode(NodeRef Node) {
     std::string NodeAttributes = DTraits.getNodeAttributes(Node, G);
 
     O << "\tNode" << static_cast<const void*>(Node) << " [shape=record,";
+    O << "fontname=\"monospace\",";
+
     if (!NodeAttributes.empty()) O << NodeAttributes << ",";
     O << "label=\"{";
 
     if (!DTraits.renderGraphFromBottomUp()) {
       O << DOT::EscapeString(DTraits.getNodeLabel(Node, G));
 
       // If we should include the address of the node in the label, do so now.
       std::string Id = DTraits.getNodeIdentifierLabel(Node, G);