[turbofan] Make --trace-turbo-graph annotate successors.

R=herhut@chromium.org

Change-Id: I052aee6778e96feeac5610c52b34b65d26de4dcb
Reviewed-on: https://chromium-review.googlesource.com/1112000Reviewed-by: Stephan Herhut <herhut@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53991}

src/compiler/instruction.cc

@@ -733,6 +733,7 @@ std::ostream& operator<<(std::ostream& os,
     os << "   " << std::setw(5) << j << ": " << printable_instr << std::endl;
   }
 
+  os << " succecessors:";
   for (RpoNumber succ : block->successors()) {
     os << " B" << succ.ToInt();
   }