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