cmd/compile: in poset, refactor aliasnode
cmd/compile: in poset, refactor aliasnode

In preparation for allowing to make multiple nodes as aliases
in a single pass, refactor aliasnode splitting out the case
in which one of the nodes is not in the post into a new
funciton (aliasnewnode).

No functional changes, passes toolstash -cmp

Change-Id: I19ca6ef8426f8aec9f2622b6151c5c617dbb25b5
Reviewed-on: https://go-review.googlesource.com/c/go/+/200859
Reviewed-by: Keith Randall <8c99c3a9284e493be632950b84cd789d08ed3e9d@golang.org>
Run-TryBot: Giovanni Bajo <387225cd66f97dc812a00599dbc5b6544a73295c@develer.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

nawawi/go,akutz/go,golang/go,akutz/go,akutz/go,nawawi/go,cloudflare/go,akutz/go,golang/go,CAFxX/go,CAFxX/go,golang/go,cloudflare/go,golang/go,golang/go,cloudflare/go,CAFxX/go,cloudflare/go,cloudflare/go,CAFxX/go,CAFxX/go,golang/go,akutz/go,akutz/go,nawawi/go,nawawi/go,akutz/go,akutz/go,cloudflare/go,cloudflare/go,nawawi/go,CAFxX/go,golang/go,nawawi/go,golang/go,cloudflare/go,nawawi/go,CAFxX/go,CAFxX/go,nawawi/go