cmd/compile/internal/liveness: use slices.Reverse

cmd/compile/internal/liveness: use slices.Reverse

Now that we're bootstrapping from a toolchain that has the slices
package.

Updates #64751

Change-Id: Id50d76de05e353ef06d64b47ad6400b2b7572205
Reviewed-on: https://go-review.googlesource.com/c/go/+/610775
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>