cmd/compile: pre-alloc enough room for Escape.walkAll

cmd/compile: pre-alloc enough room for Escape.walkAll

Slightly reduce allocs, passes toolstash-check.

name          old time/op       new time/op       delta
Template            181ms ± 4%        174ms ± 0%  -3.59%  (p=0.008 n=5+5)

name          old user-time/op  new user-time/op  delta
Template            249ms ± 3%        240ms ± 2%  -3.59%  (p=0.016 n=5+5)

name          old alloc/op      new alloc/op      delta
Template           35.0MB ± 0%       34.9MB ± 0%  -0.09%  (p=0.008 n=5+5)
Unicode            28.6MB ± 0%       28.6MB ± 0%    ~     (p=0.421 n=5+5)
GoTypes             114MB ± 0%        114MB ± 0%  -0.10%  (p=0.008 n=5+5)
Compiler            542MB ± 0%        541MB ± 0%  -0.13%  (p=0.008 n=5+5)
SSA                1.21GB ± 0%       1.21GB ± 0%  -0.10%  (p=0.008 n=5+5)
Flate              22.0MB ± 0%       22.0MB ± 0%  -0.05%  (p=0.016 n=5+5)
GoParser           27.1MB ± 0%       27.0MB ± 0%  -0.05%  (p=0.008 n=5+5)
Reflect            74.8MB ± 0%       74.8MB ± 0%  -0.11%  (p=0.008 n=5+5)
Tar                33.0MB ± 0%       32.9MB ± 0%  -0.07%  (p=0.008 n=5+5)
XML                42.1MB ± 0%       42.1MB ± 0%  -0.07%  (p=0.008 n=5+5)
LinkCompiler        222MB ± 0%        222MB ± 0%    ~     (p=0.690 n=5+5)
[Geo mean]         81.3MB            81.2MB       -0.07%

name          old allocs/op     new allocs/op     delta
Template             347k ± 0%         347k ± 0%  -0.16%  (p=0.008 n=5+5)
Unicode              334k ± 0%         334k ± 0%  -0.03%  (p=0.016 n=5+5)
GoTypes             1.20M ± 0%        1.20M ± 0%  -0.12%  (p=0.008 n=5+5)
Compiler            5.13M ± 0%        5.12M ± 0%  -0.11%  (p=0.008 n=5+5)
SSA                 11.7M ± 0%        11.7M ± 0%  -0.13%  (p=0.008 n=5+5)
Flate                221k ± 0%         221k ± 0%  -0.18%  (p=0.008 n=5+5)
GoParser             280k ± 0%         280k ± 0%  -0.06%  (p=0.008 n=5+5)
Reflect              902k ± 0%         900k ± 0%  -0.28%  (p=0.008 n=5+5)
Tar                  323k ± 0%         322k ± 0%  -0.18%  (p=0.008 n=5+5)
XML                  401k ± 0%         401k ± 0%  -0.10%  (p=0.008 n=5+5)
LinkCompiler         735k ± 0%         735k ± 0%    ~     (p=0.841 n=5+5)
[Geo mean]           753k              752k       -0.12%

Change-Id: I647bd7752f28b74e6f400fa16cb69632f5c952b3
Reviewed-on: https://go-review.googlesource.com/c/go/+/229517
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>