cmd/compile: refactor appendslice to use newer gc code style

cmd/compile: refactor appendslice to use newer gc code style

- add comments with builtin function signatures that are instantiated
- use Nodes type from the beginning instead of
  []*Node with a later conversion to Nodes
- use conv(x, y) helper function instead of nod(OCONV, x, y)
- factor out repeated calls to Type.Elem()

This makes the function style similar to newer functions like extendslice.

passes toolstash -cmp

Change-Id: Iedab191af9e0884fb6762c9c168430c1d2246979
Reviewed-on: https://go-review.googlesource.com/112598
Run-TryBot: Martin Möhrmann <moehrmann@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>