The macro pn ensures that TurboFan nodes can be easily
printed in gdb, even in release builds where Node::Print
is sometimes not available (because all uses have been
inlined).

This CL also modifies the print function to deal gracefully
with nullptr input nodes, which is helpful for debugging.

Change-Id: Ib5f58aa13b719c8390826bc89dfe21cf58586de5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1672941Reviewed-by: Georg Neis <neis@chromium.org>
Reviewed-by: Tamer Tas <tmrts@chromium.org>
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62422}