cmd/compile: some cleanup with old irgen code

cmd/compile: some cleanup with old irgen code

 - Un-export Convertop: it's only used by tcConv.
 - Remove AssignOp1: introduced in CL 349614, only used by irgen.
 - Un-export Assignop: it was exported to be used by irgen only.

Change-Id: I7e78b35d90f165c537cf32a104156bf2a13ca8b8
Reviewed-on: https://go-review.googlesource.com/c/go/+/532516
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>