[dev.typeparams] cmd/compile: don't need to unshapify append calls

append is fine using shape types.

Change-Id: Iae829b9b5929d4dc7aa74bed57da13d4f6d746be
Reviewed-on: https://go-review.googlesource.com/c/go/+/337669


Trust: Keith Randall <khr@golang.org>
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dan Scales <danscales@google.com>