cmd/compile: eliminate all references to Curfn in liveness

Updates #15756

Change-Id: I5ad87ef44b8ee48e1294820e0b1ab0ec07c480eb
Reviewed-on: https://go-review.googlesource.com/38590


Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>