[dev.regabi] cmd/compile/internal/gc: refactor to use stop using Func.Nname

Automated factoring produced by rf script below to replace uses of
Func.Nname with Field.Nname or Node.MethodName as appropriate.

Some dead assignments to Func.Nname are left behind; these will be
removed in a subequent remove-only CL.

Passes toolstash-check.

[git-generate]
cd src/cmd/compile/internal/gc
rf '
ex \
  import "cmd/compile/internal/types"; \
  var f *types.Field; \
  var n *types.Node; \
  f.Type.Nname() -> f.Nname; \
  f.Type.SetNname(n) -> f.Nname = n; \
  f.Type.FuncType().Nname -> f.Nname

ex \
  var n *Node; \
  asNode(n.Type.Nname()) -> n.MethodName(); \
  asNode(n.Type.FuncType().Nname) -> n.MethodName(); \
  asNode(callpartMethod(n).Type.Nname()) -> n.MethodName()
'

Change-Id: Iaae054324dfe7da6f5d8b8d57a1e05b58cc5968c
Reviewed-on: https://go-review.googlesource.com/c/go/+/272389


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>