cmd/compile: skip diag error in checkassign if one was emitted

While at it, also remove checkassignlist, which is not used.

For #43311

Change-Id: Ie7ed81f68080d8881fca6035da64a9755f2cb555
Reviewed-on: https://go-review.googlesource.com/c/go/+/294032
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

@@ -1612,6 +1612,10 @@ func checkassign(stmt ir.Node, n ir.Node) {
 		return
 	}
 
+	defer n.SetType(nil)
+	if n.Diag() {
+		return
+	}
 	switch {
 	case n.Op() == ir.ODOT && n.(*ir.SelectorExpr).X.Op() == ir.OINDEXMAP:
 		base.Errorf("cannot assign to struct field %v in map", n)
@@ -1622,13 +1626,6 @@ func checkassign(stmt ir.Node, n ir.Node) {
 	default:
 		base.Errorf("cannot assign to %v", n)
 	}
-	n.SetType(nil)
-}
-
-func checkassignlist(stmt ir.Node, l ir.Nodes) {
-	for _, n := range l {
-		checkassign(stmt, n)
-	}
 }
 
 func checkassignto(src *types.Type, dst ir.Node) {