[dev.typeparams] cmd/compile: fail early on unexpected types2.Invalid

In unified IR, fail right away if we find a types2.Invalid while
writing out the package. This provides a clearer error message for
https://github.com/golang/go/issues/25838#issuecomment-448746670.

Updates #25838.

Change-Id: I6902fdd891fc31bbb832b6fdba00eca301282409
Reviewed-on: https://go-review.googlesource.com/c/go/+/338973


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/cmd/compile/internal/noder/writer.go

@@ -282,18 +282,22 @@ func (pw *pkgWriter) typIdx(typ types2.Type, dict *writerDict) typeInfo {
 		base.Fatalf("unexpected type: %v (%T)", typ, typ)
 
 	case *types2.Basic:
-		if kind := typ.Kind(); types2.Typ[kind] == typ {
+		switch kind := typ.Kind(); {
+		case kind == types2.Invalid:
+			base.Fatalf("unexpected types2.Invalid")
+
+		case types2.Typ[kind] == typ:
 			w.code(typeBasic)
 			w.len(int(kind))
-			break
-		}
 
-		// Handle "byte" and "rune" as references to their TypeName.
-		obj := types2.Universe.Lookup(typ.Name())
-		assert(obj.Type() == typ)
+		default:
+			// Handle "byte" and "rune" as references to their TypeName.
+			obj := types2.Universe.Lookup(typ.Name())
+			assert(obj.Type() == typ)
 
-		w.code(typeNamed)
-		w.obj(obj, nil)
+			w.code(typeNamed)
+			w.obj(obj, nil)
+		}
 
 	case *types2.Named:
 		// Type aliases can refer to uninstantiated generic types, so we