cmd/compile: prevent modification of ONAME/OLITERAL/OTYPES nodes in walkexpr

ONAME, OLITERAL, and OTYPE nodes can be shared between functions.
In a concurrent backend, such nodes might be walked concurrently
with being read in other functions.
Arrange for them to be unmodified by walk.

This is a follow-up to CL 38609.

Passes toolstash-check.

Updates #15756

Change-Id: I03ff1d2c0ad81dafac3fd55caa218939cf7c0565
Reviewed-on: https://go-review.googlesource.com/38655
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

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

@@ -1181,7 +1181,11 @@ func updateHasCall(n *Node) {
 	}
 
 	switch n.Op {
-	case OLITERAL, ONAME:
+	case OLITERAL, ONAME, OTYPE:
+		if b || n.HasCall() {
+			Fatalf("OLITERAL/ONAME/OTYPE should never have calls: %+v", n)
+		}
+		return
 	case OAS:
 		if needwritebarrier(n.Left) {
 			b = true

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

@@ -498,7 +498,10 @@ opswitch:
 		Dump("walk", n)
 		Fatalf("walkexpr: switch 1 unknown op %+S", n)
 
-	case OTYPE, ONONAME, OINDREGSP, OEMPTY, OGETG:
+	case ONONAME, OINDREGSP, OEMPTY, OGETG:
+
+	case OTYPE, ONAME, OLITERAL:
+		// TODO(mdempsky): Just return n; see discussion on CL 38655.
 
 	case ONOT, OMINUS, OPLUS, OCOM, OREAL, OIMAG, ODOTMETH, ODOTINTER,
 		OIND, OSPTR, OITAB, OIDATA, ODOTTYPE, ODOTTYPE2, OADDR:
@@ -593,7 +596,7 @@ opswitch:
 	case ORECOVER:
 		n = mkcall("gorecover", n.Type, init, nod(OADDR, nodfp, nil))
 
-	case OLITERAL, OCLOSUREVAR, OCFUNC, ONAME:
+	case OCLOSUREVAR, OCFUNC:
 		n.SetAddable(true)
 
 	case OCALLINTER: