cmd/compile/internal/typecheck: avoid use of Name.Ntype for assignments

Prep refactoring for the next CL, which removes Name.Ntype
entirely. Pulled out separately because this logic is a little subtle,
so this should be easier to bisect in case there's something I'm
missing here.

Change-Id: I4ffec6ee62fcd036582e8d2c963edcbd8bac184f
Reviewed-on: https://go-review.googlesource.com/c/go/+/403837
Reviewed-by: David Chase <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Cuong Manh Le <[email protected]>
Run-TryBot: Matthew Dempsky <[email protected]>

Author: mdempsky

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

@@ -127,12 +127,9 @@ func assign(stmt ir.Node, lhs, rhs []ir.Node) {
 
 	checkLHS := func(i int, typ *types.Type) {
 		lhs[i] = Resolve(lhs[i])
-		if n := lhs[i]; typ != nil && ir.DeclaredBy(n, stmt) && n.Name().Ntype == nil {
-			if typ.Kind() != types.TNIL {
-				n.SetType(defaultType(typ))
-			} else {
-				base.Errorf("use of untyped nil")
-			}
+		if n := lhs[i]; typ != nil && ir.DeclaredBy(n, stmt) && n.Type() == nil {
+			base.Assertf(typ.Kind() == types.TNIL, "unexpected untyped nil")
+			n.SetType(defaultType(typ))
 		}
 		if lhs[i].Typecheck() == 0 {
 			lhs[i] = AssignExpr(lhs[i])