cmd/compile: handle ODDD in exprformat

Fixes #41247

Change-Id: Iaa9502cc610e2cc64be5dfd91ba3187f86f87cbd
Reviewed-on: https://go-review.googlesource.com/c/go/+/252942
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

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

@@ -1616,7 +1616,8 @@ func (n *Node) exprfmt(s fmt.State, prec int, mode fmtMode) {
 			}
 			n1.exprfmt(s, nprec, mode)
 		}
-
+	case ODDD:
+		mode.Fprintf(s, "...")
 	default:
 		mode.Fprintf(s, "<node %v>", n.Op)
 	}

test/fixedbugs/issue41247.go

@@ -0,0 +1,11 @@
+// errorcheck
+
+// Copyright 2020 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() [2]int {
+	return [...]int{2: 0} // ERROR "cannot use \[\.\.\.\]int literal \(type \[3\]int\)"
+}