cmd/compile: avoid appends and allocate full slice length in copyRet

cmd/compile: avoid appends and allocate full slice length in copyRet

passes toolstash -cmp

compilebench allocs:
name      old allocs/op   new allocs/op   delta
Template       385k ± 0%       385k ± 0%  -0.00%  (p=0.017 n=19+20)
Unicode        342k ± 0%       342k ± 0%    ~     (p=0.867 n=20+20)
GoTypes       1.15M ± 0%      1.15M ± 0%  -0.00%  (p=0.008 n=20+20)
SSA           12.1M ± 0%      12.1M ± 0%    ~     (p=0.141 n=20+19)
Flate          234k ± 0%       234k ± 0%    ~     (p=0.125 n=20+19)
GoParser       315k ± 0%       315k ± 0%    ~     (p=0.104 n=20+20)
Reflect        972k ± 0%       972k ± 0%  -0.00%  (p=0.000 n=17+20)
Tar            391k ± 0%       391k ± 0%  -0.01%  (p=0.000 n=19+20)
XML            404k ± 0%       403k ± 0%  -0.01%  (p=0.000 n=20+19)

Change-Id: Ie24f7fae7b6b85422ec1ff0240f08f0a34064d72
Reviewed-on: https://go-review.googlesource.com/c/144038
Run-TryBot: Martin Möhrmann <martisch@uos.de>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>