cmd/compile: remove "special return in disguise" case

ascompatee does not generate 'x = x' during return, so we don't have to
check for samelist and disguising special return anymore.

While at it, also remove samelist, as this is the only place it's used.

Passes toolstash-check.

Change-Id: I41c7b077d562aadb5916a61e2ab6229bae3cdef4
Reviewed-on: https://go-review.googlesource.com/c/go/+/227807


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