cmd/compile: fix nodedump output for types of nodes

The Dbg dumping of complex types was broken, because (I think) of a
recent change to handle recursive types correctly. Before this fix,
the Dump output of a closure node (where the last thing on the line is
the type of the node) was:

.   .   CLOSURE l(8) esc(h) tc(1) FUNC-@0

after this change it is:

.   .   CLOSURE l(8) esc(h) tc(1) FUNC-func(int) int

The problem is that that the 'mode == Fdbg' code was immediately
aborting the descent into tconv2, since it was calling down with the
same node that was just entered into the hash table.

Change-Id: Iee106b967cea1856dd92d4350681401dd34a23b3
Reviewed-on: https://go-review.googlesource.com/c/go/+/264025


Trust: Dan Scales <danscales@google.com>
Reviewed-by: Keith Randall <khr@golang.org>