cmd/compile: remove some uses of ir.CurFunc

cmd/compile: remove some uses of ir.CurFunc

This CL updates several frontend passes to stop relying on
ir.CurFunc (at least directly).

Change-Id: I3c3529e81e27fb05d54a828f081f7c7efc31af67
Reviewed-on: https://go-review.googlesource.com/c/go/+/520606
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Matthew Dempsky <mdempsky@google.com>