[dev.regabi] cmd/compile: remove typ from AssignOpStmt

Previous detached logic of typechecking AssignOpStmt from tcArith, the
typ field of it is not used anymore.

Pass toolstash -cmp.

Change-Id: I407507a1c4c4f2958fca4d6899875564e54bf1f5
Reviewed-on: https://go-review.googlesource.com/c/go/+/279443


Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>