cmd/compile: sort liveness variable reports

cmd/compile: sort liveness variable reports

Sort variables before display so that when there are multiple variables
to report, they are in a consistent order.

Otherwise they are ordered in the order they appear in the fn.Dcl list,
which can vary. Particularly, they vary depending on regabi.

Change-Id: I0db380f7cbe6911e87177503a4c3b39851ff1b5a
Reviewed-on: https://go-review.googlesource.com/c/go/+/462898
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>