[dev.regabi] cmd/compile: fix ICE during ir.Dump

fmt.go:dumpNodeHeader uses reflection to call all "func() bool"-typed
methods on Nodes during printing, but the OnStack method that I added
in CL 283233 isn't meant to be called on non-variables.

dumpNodeHeader does already guard against panics, as happen in some
other accessors, but not against Fatalf, as I was using in OnStack. So
simply change OnStack to use panic too.

Thanks to drchase@ for the report.

Change-Id: I0cfac84a96292193401a32fc5e7fd3c48773e008
Reviewed-on: https://go-review.googlesource.com/c/go/+/284074
Trust: Matthew Dempsky <[email protected]>
Run-TryBot: Matthew Dempsky <[email protected]>
Reviewed-by: David Chase <[email protected]>
TryBot-Result: Go Bot <[email protected]>

Author: mdempsky

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

@@ -286,18 +286,17 @@ func (n *Name) SetLibfuzzerExtraCounter(b bool) { n.flags.set(nameLibfuzzerExtra
 
 // OnStack reports whether variable n may reside on the stack.
 func (n *Name) OnStack() bool {
-	if n.Op() != ONAME || n.Class == PFUNC {
-		base.Fatalf("%v is not a variable", n)
-	}
-	switch n.Class {
-	case PPARAM, PPARAMOUT, PAUTO:
-		return n.Esc() != EscHeap
-	case PEXTERN, PAUTOHEAP:
-		return false
-	default:
-		base.FatalfAt(n.Pos(), "%v has unknown class %v", n, n.Class)
-		panic("unreachable")
+	if n.Op() == ONAME {
+		switch n.Class {
+		case PPARAM, PPARAMOUT, PAUTO:
+			return n.Esc() != EscHeap
+		case PEXTERN, PAUTOHEAP:
+			return false
+		}
 	}
+	// Note: fmt.go:dumpNodeHeader calls all "func() bool"-typed
+	// methods, but it can only recover from panics, not Fatalf.
+	panic(fmt.Sprintf("%v: not a variable: %v", base.FmtPos(n.Pos()), n))
 }
 
 // MarkReadonly indicates that n is an ONAME with readonly contents.