cmd/compile/internal/staticinit: fix panic in interface conversion

This patch fixes a panic from incorrect interface conversion
from *ir.BasicLit to *ir.ConstExpr. This only occurs when nounified
GOEXPERIMENT is set, so ideally it should be backported to Go 1.20
and removed from master.

Fixes #58339

Author: sywhang

src/cmd/compile/internal/staticinit/sched.go

@@ -870,8 +870,10 @@ func subst(n ir.Node, m map[*ir.Name]ir.Node) (ir.Node, bool) {
 			// on variables are not valid on constants, so we might have
 			// generated invalid code that will trip up the rest of the compiler.
 			// Fix those by truncating the constants.
-			if x, ok := truncate(x.X.(*ir.ConstExpr), x.Type()); ok {
-				return x
+			if ce, ok := x.X.(*ir.ConstExpr); ok {
+				if x, ok := truncate(ce, x.Type()); ok {
+					return x
+				}
 			}
 			valid = false
 			return x

test/fixedbugs/issue58339.dir/a.go

@@ -0,0 +1,17 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package a
+
+func Assert(msgAndArgs ...any) {
+}
+
+func Run() int {
+	Assert("%v")
+	return 0
+}
+
+func Run2() int {
+	return Run()
+}

test/fixedbugs/issue58339.dir/b.go

@@ -0,0 +1,9 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package b
+
+import "./a"
+
+var A = a.Run2()

test/fixedbugs/issue58339.go

@@ -0,0 +1,7 @@
+// compiledir
+
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ignored