cmd/compile: make LivenessMap sparse

We're about to switch to having significantly fewer maps in the
liveness map, so switch from a dense representation to a sparse
representation.

Passes toolstash-check.

For #36365.

Change-Id: Icb17bd6ace17667a280bc5fba4039cae3020a8d1
Reviewed-on: https://go-review.googlesource.com/c/go/+/230543


Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>