Highlight incoming/outgoing edges on hover in HLO graphviz dumps, and other improvements.

Other improvements:

 - Don't show tooltips for nodes and clusters.  Previously we'd show a
   tooltip containing a pointer value expressed as decimal.  Not so
   useful.

 - Show tooltips on edges with the to/from node names.

 - Fix bug wherein if we had

   - a node at the "edge" of the graph (so its operands aren't included
     unless they're referenced by another node),
   - with all of its operands included in the graph save one or more
     constants, and
   - those constants weren't referenced by any nodes not at the edge of
     the graph,

   we would incorrectly draw the node as "grayed out", indicating that
   one of its operands (namely, its constant operand) wasn't present in
   the graph.

   This is wrong because constants are inlined into their users, so they
   should always count as "displayed" for the purposes of determining
   whether a node is grayed out.

PiperOrigin-RevId: 163276108