[dev.regabi] cmd/compile: simplify Nodes usage [generated]

Now that Nodes is a slice, most of the methods can be removed
in favor of direct slice operations, reducing the new API that must
be understood to:

	Copy
	Take
	Append
	Prepend
	Format

Passes buildall w/ toolstash -cmp.

[git-generate]
cd src/cmd/compile/internal/ir
rf '
ex . ../gc {
	var ns Nodes
	var pns *Nodes
	var n, n2, n3 Node
	var i int
	var slice []Node

	ns.Len() -> len(ns)
	ns.Slice() -> ns
	ns.First() -> ns[0]
	ns.Second() -> ns[1]
	ns.Index(i) -> ns[i]
	ns.Addr(i) -> &ns[i]
	ns.SetIndex(i, n) -> ns[i] = n
	ns.SetFirst(n) -> ns[0] = n
	ns.SetSecond(n) -> ns[1] = n
	ns.Set1(n) -> ns = []Node{n}
	ns.Set2(n, n2) -> ns = []Node{n, n2}
	ns.Set3(n, n2, n3) -> ns = []Node{n, n2, n3}
	ns.Set1(n) -> ns = []Node{n}
	ns.Set2(n, n2) -> ns = []Node{n, n2}
	ns.Set3(n, n2, n3) -> ns = []Node{n, n2, n3}
	AsNodes(slice) -> Nodes(slice)
	ns.AppendNodes(pns) -> ns.Append(pns.Take()...)
	ns.MoveNodes(pns) -> ns = pns.Take()
}

rm \
	Nodes.Len Nodes.Slice \
	Nodes.First Nodes.Second Nodes.Index Nodes.Addr \
	Nodes.SetIndex Nodes.SetFirst Nodes.SetSecond \
	Nodes.Set1 Nodes.Set2 Nodes.Set3 \
	AsNodes \
	Nodes.AppendNodes Nodes.MoveNodes
'

Change-Id: Iee86434ced52e67861c3fa71bdd6d994a8cba735
Reviewed-on: https://go-review.googlesource.com/c/go/+/277936


Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>