cmd/compile: flag "-d=dumpptrs" to print Node ptrs in Dump output

The printing of the ptr values can mean that two dump outputs can't easily be
compared for the identical structure, so adding the "-d=dumpptrs" option to make
printing of Node pointer values be an option.

Change-Id: I0e92b02f069e9de2e6fa036a7841645d13cdd7a9
Reviewed-on: https://go-review.googlesource.com/c/go/+/271339


Trust: Dan Scales <danscales@google.com>
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>