Currently Liveness.compact rewrites the Liveness.livevars slice in
place. However, we're about to add register maps, which we'll want to
track in livevars, but compact independently from the stack maps.
Hence, this CL modifies Liveness.compact to consume Liveness.livevars
and produce a new slice of deduplicated stack maps. This is somewhat
clearer anyway because it avoids potential confusion over how
Liveness.livevars is indexed.

Passes toolstash -cmp.

For #24543.

Change-Id: I7093fbc71143f8a29e677aa30c96e501f953ca2b
Reviewed-on: https://go-review.googlesource.com/108498
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>