cmd/compile: do not set type for OPACK

That's an invalid operation and depend on gc.hidePanic to report error.

Updates #43311

Change-Id: I78d615c40ab1e7887f612491e215c1c2bb758ef6
Reviewed-on: https://go-review.googlesource.com/c/go/+/294031
Trust: Cuong Manh Le <[email protected]>
Run-TryBot: Cuong Manh Le <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Matthew Dempsky <[email protected]>

Author: cuonglm

src/cmd/compile/internal/typecheck/typecheck.go

@@ -528,7 +528,7 @@ func typecheck1(n ir.Node, top int) ir.Node {
 	case ir.OPACK:
 		n := n.(*ir.PkgName)
 		base.Errorf("use of package %v without selector", n.Sym())
-		n.SetType(nil)
+		n.SetDiag(true)
 		return n
 
 	// types (ODEREF is with exprs)