[dev.link] cmd/link: reuse slice memory in deadcode pass

[dev.link] cmd/link: reuse slice memory in deadcode pass

Reuse slice memory in the deadcode pass, reduce allocations.

Linking cmd/compile,

name           old alloc/op   new alloc/op   delta
Deadcode_GC      2.10MB ± 0%    1.41MB ± 0%  -32.61%  (p=0.008 n=5+5)

name           old allocs/op  new allocs/op  delta
Deadcode_GC       8.46k ± 0%     5.55k ± 0%  -34.45%  (p=0.008 n=5+5)

Change-Id: Ib9ba0928d68a65879007218697712b53acd3c5c5
Reviewed-on: https://go-review.googlesource.com/c/go/+/236566
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jeremy Faller <jeremy@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>