cmd/compile: don't mutate shared nodes in orderinit

A few gc.Node ops may be shared across functions.
The compiler is (mostly) already careful to avoid mutating them.
However, from a concurrency perspective, replacing (say)
an empty list with an empty list still counts as a mutation.
One place this occurs is orderinit. Avoid it.

This requires fixing one spot where shared nodes were mutated.
It doesn't result in any functional or performance changes.

Passes toolstash-check.

Updates #15756

Change-Id: I63c93b31baeeac62d7574804acb6b7f2bc9d14a9
Reviewed-on: https://go-review.googlesource.com/39196
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/cmd/compile/internal/gc/syntax.go

@@ -14,7 +14,7 @@ import (
 // A Node is a single node in the syntax tree.
 // Actually the syntax tree is a syntax DAG, because there is only one
 // node with Op=ONAME for a given instance of a variable x.
-// The same is true for Op=OTYPE and Op=OLITERAL.
+// The same is true for Op=OTYPE and Op=OLITERAL. See Node.mayBeShared.
 type Node struct {
 	// Tree structure.
 	// Generic recursive walks should follow these fields.
@@ -179,6 +179,16 @@ func (n *Node) SetIota(x int64) {
 	n.Xoffset = x
 }
 
+// mayBeShared reports whether n may occur in multiple places in the AST.
+// Extra care must be taken when mutating such a node.
+func (n *Node) mayBeShared() bool {
+	switch n.Op {
+	case ONAME, OLITERAL, OTYPE:
+		return true
+	}
+	return false
+}
+
 // Name holds Node fields used only by named nodes (ONAME, OTYPE, OPACK, OLABEL, some OLITERAL).
 type Name struct {
 	Pack      *Node  // real package for import . names