cmd/compile/internal/devirtualize: do not track assignments to non-PAUTO

We do not lookup/devirtualize such, so we can skip tracking them.

Change-Id: I8bdb0b11c694e4b2326c236093508a356a6a6964
Reviewed-on: https://go-review.googlesource.com/c/go/+/711160
Reviewed-by: Keith Randall <khr@golang.org>
Auto-Submit: Keith Randall <khr@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>

Author: mateusz834

src/cmd/compile/internal/devirtualize/devirtualize.go

@@ -387,6 +387,9 @@ func (s *State) assignments(n *ir.Name) []assignment {
 	if fun == nil {
 		base.FatalfAt(n.Pos(), "n.Curfn = <nil>")
 	}
+	if n.Class != ir.PAUTO {
+		base.FatalfAt(n.Pos(), "n.Class = %v; want = PAUTO", n.Class)
+	}
 
 	if !n.Type().IsInterface() {
 		base.FatalfAt(n.Pos(), "name passed to assignments is not of an interface type: %v", n.Type())
@@ -432,6 +435,9 @@ func (s *State) analyze(nodes ir.Nodes) {
 		if n.Op() != ir.ONAME {
 			base.FatalfAt(n.Pos(), "n.Op = %v; want = ONAME", n.Op())
 		}
+		if n.Class != ir.PAUTO {
+			return nil, -1
+		}
 
 		switch a := assignment.(type) {
 		case nil: