[dev.regabi] cmd/compile/internal/gc: prep for Func.Nname removal refactoring

There are three bits of method-handling code where we separately go
from Field->Type and then Type->Node. By shuffling the code around a
little to go Field->Type->Node in a single statement, we're able to
more easily remove Type from the operation.

Passes toolstash-check.

Change-Id: Ife98216d70d3b867fa153449abef0e56a4fb242a
Reviewed-on: https://go-review.googlesource.com/c/go/+/272388


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>