compiler: set varargs correctly for type of method expression

Fixes golang/go#41737

Change-Id: I080886fef85d9e00af3b20d4fd0e38d8cfe74725
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/258977
Trust: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>