gc: line number + type checking nits

Fixes #1910.
Fixes #1979.
Fixes #1990.
Fixes #1993.
Fixes #2089.

R=ken2
CC=golang-dev
https://golang.org/cl/4828046

test/fixedbugs/bug353.go

@@ -0,0 +1,30 @@
+// errchk $G $D/$F.go
+
+// Copyright 2011 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.
+
+// issue 2089 - internal compiler error
+
+package main
+
+import (
+	"io"
+	"os"
+)
+
+func echo(fd io.ReadWriterCloser) { // ERROR "undefined: io.ReadWriterCloser"
+	var buf [1024]byte
+	for {
+		n, err := fd.Read(buf)
+		if err != nil {
+			break
+		}
+		fd.Write(buf[0:n])
+	}
+}
+
+func main() {
+	fd, _ := os.Open("a.txt")
+	echo(fd)
+}