[dev.regabi] cmd/compile: implement copy for nodes

Put each node in charge of making copies of its own slices.
This removes a generic use of Body, SetBody, and so on
in func Copy, heading toward removing those even from
being used in package ir.

Passes buildall w/ toolstash -cmp.

Change-Id: I249b7fe54cf72e9d2f0467b10f3f257abf9b29b9
Reviewed-on: https://go-review.googlesource.com/c/go/+/275374
Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/ir/node.go

@@ -27,8 +27,8 @@ type Node interface {
 	Pos() src.XPos
 	SetPos(x src.XPos)
 
-	// For making copies. Mainly used by Copy and SepCopy.
-	rawCopy() Node
+	// For making copies. For Copy and SepCopy.
+	copy() Node
 
 	// Abstract graph structure, for generic traversals.
 	Op() Op
@@ -521,6 +521,21 @@ func (n *Nodes) AppendNodes(n2 *Nodes) {
 	n2.slice = nil
 }
 
+// Copy returns a copy of the content of the slice.
+func (n Nodes) Copy() Nodes {
+	var c Nodes
+	if n.slice == nil {
+		return c
+	}
+	c.slice = new([]Node)
+	if *n.slice == nil {
+		return c
+	}
+	*c.slice = make([]Node, n.Len())
+	copy(*c.slice, n.Slice())
+	return c
+}
+
 // nodeQueue is a FIFO queue of *Node. The zero value of nodeQueue is
 // a ready-to-use empty queue.
 type NodeQueue struct {