cmd/compile: don't set Ntype in noder2 anymore

Now that we are no longer calling the old typechecker at all during the
noder2 pass, we don't need to create and set an Ntype node ((which is
just a node representation of the type which we already know) for the
Name and Closure nodes. This should reduce memory usage a bit for -G=3.

Change-Id: I6b1345007ce067a89ee64955a53f25645c303f4d
Reviewed-on: https://go-review.googlesource.com/c/go/+/308909


Trust: Dan Scales <danscales@google.com>
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>