cmd/compile: avoid duplicate cast error

If an error was already printed during LHS conversion step, we don't reprint
the "cannot convert" error.

In particular, this prevents `_ = int("1")` (and all similar casts) from
resulting in multiple identical error messages being printed.

Fixes #20812.

Change-Id: If6e52c59eab438599d641ecf6f110ebafca740a9
Reviewed-on: https://go-review.googlesource.com/46912Reviewed-by: Robert Griesemer <gri@golang.org>

src/cmd/compile/internal/gc/const.go

@@ -408,7 +408,7 @@ func convlit1(n *Node, t *types.Type, explicit bool, reuse canReuseNode) *Node {
 bad:
 	if !n.Diag() {
 		if !t.Broke() {
-			yyerror("cannot convert %v to type %v", n, t)
+			yyerror("cannot convert %L to type %v", n, t)
 		}
 		n.SetDiag(true)
 	}

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

@@ -1760,7 +1760,7 @@ OpSwitch:
 		var why string
 		n.Op = convertop(t, n.Type, &why)
 		if n.Op == 0 {
-			if !n.Diag() && !n.Type.Broke() {
+			if !n.Diag() && !n.Type.Broke() && !n.Left.Diag() {
 				yyerror("cannot convert %L to type %v%s", n.Left, n.Type, why)
 				n.SetDiag(true)
 			}

test/fixedbugs/issue20812.go

+// errorcheck
+
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+func f() {
+	_ = int("1")      // ERROR "cannot convert"
+	_ = bool(0)       // ERROR "cannot convert"
+	_ = bool("false") // ERROR "cannot convert"
+	_ = int(false)    // ERROR "cannot convert"
+	_ = string(true)  // ERROR "cannot convert"
+}

test/rename1.go

@@ -13,7 +13,7 @@ func main() {
 	var n byte         // ERROR "not a type|expected type"
 	var y = float32(0) // ERROR "cannot call|expected function"
 	const (
-		a = 1 + iota // ERROR "string|incompatible types" "convert iota"
+		a = 1 + iota // ERROR "invalid operation|incompatible types" "cannot convert iota"
 	)
 
 }