cmd/compile: do not mark export for send only chan

No changes in compilebench, pass toolstash-check.

Change-Id: I2688f7b45af0eaa0cf3b38e726bce6e68c20f69c
Reviewed-on: https://go-review.googlesource.com/c/go/+/195077
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

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

@@ -43,11 +43,14 @@ func (p *exporter) markType(t *types.Type) {
 	// the user already needs some way to construct values of
 	// those types.
 	switch t.Etype {
-	case TPTR, TARRAY, TSLICE, TCHAN:
-		// TODO(mdempsky): Skip marking element type for
-		// send-only channels?
+	case TPTR, TARRAY, TSLICE:
 		p.markType(t.Elem())
 
+	case TCHAN:
+		if t.ChanDir().CanRecv() {
+			p.markType(t.Elem())
+		}
+
 	case TMAP:
 		p.markType(t.Key())
 		p.markType(t.Elem())