Allocate two more ssa local worklists on the stack. The initial sizes
are chosen to cover >99% of the calls.

name      old time/op       new time/op       delta
Template        281ms ± 2%        283ms ± 5%    ~     (p=0.443 n=18+19)
Unicode         136ms ± 4%        135ms ± 7%    ~     (p=0.277 n=20+20)
GoTypes         886ms ± 2%        885ms ± 2%    ~     (p=0.862 n=20+20)
Compiler        4.03s ± 2%        4.02s ± 1%    ~     (p=0.270 n=19+20)
SSA             9.66s ± 1%        9.64s ± 2%    ~     (p=0.253 n=20+20)
Flate           186ms ± 5%        183ms ± 6%    ~     (p=0.174 n=20+20)
GoParser        222ms ± 4%        219ms ± 4%    ~     (p=0.081 n=20+20)
Reflect         569ms ± 2%        568ms ± 2%    ~     (p=0.686 n=19+19)
Tar             258ms ± 4%        256ms ± 3%    ~     (p=0.211 n=20+20)
XML             319ms ± 2%        317ms ± 3%    ~     (p=0.158 n=18+20)

name      old user-time/op  new user-time/op  delta
Template        396ms ± 6%        392ms ± 6%    ~     (p=0.211 n=20+20)
Unicode         212ms ±10%        211ms ± 9%    ~     (p=0.904 n=20+20)
GoTypes         1.21s ± 3%        1.21s ± 2%    ~     (p=0.183 n=20+20)
Compiler        5.60s ± 2%        5.62s ± 2%    ~     (p=0.355 n=18+18)
SSA             14.0s ± 6%        13.9s ± 5%    ~     (p=0.678 n=20+20)
Flate           250ms ± 8%        245ms ± 6%    ~     (p=0.166 n=19+20)
GoParser        305ms ± 6%        304ms ± 5%    ~     (p=0.659 n=20+20)
Reflect         760ms ± 3%        758ms ± 4%    ~     (p=0.758 n=20+20)
Tar             362ms ± 6%        357ms ± 5%    ~     (p=0.108 n=20+20)
XML             429ms ± 4%        429ms ± 4%    ~     (p=0.799 n=20+20)

name      old alloc/op      new alloc/op      delta
Template       39.0MB ± 0%       38.8MB ± 0%  -0.55%  (p=0.000 n=20+20)
Unicode        29.1MB ± 0%       29.1MB ± 0%  -0.06%  (p=0.000 n=20+20)
GoTypes         116MB ± 0%        115MB ± 0%  -0.50%  (p=0.000 n=20+20)
Compiler        493MB ± 0%        491MB ± 0%  -0.46%  (p=0.000 n=19+20)
SSA            1.40GB ± 0%       1.40GB ± 0%  -0.31%  (p=0.000 n=19+20)
Flate          25.0MB ± 0%       24.9MB ± 0%  -0.60%  (p=0.000 n=19+19)
GoParser       30.9MB ± 0%       30.7MB ± 0%  -0.66%  (p=0.000 n=20+20)
Reflect        77.5MB ± 0%       77.1MB ± 0%  -0.52%  (p=0.000 n=20+20)
Tar            39.2MB ± 0%       39.0MB ± 0%  -0.47%  (p=0.000 n=20+20)
XML            44.8MB ± 0%       44.6MB ± 0%  -0.45%  (p=0.000 n=20+19)

name      old allocs/op     new allocs/op     delta
Template         382k ± 0%         379k ± 0%  -0.69%  (p=0.000 n=20+19)
Unicode          337k ± 0%         336k ± 0%  -0.09%  (p=0.000 n=20+20)
GoTypes         1.19M ± 0%        1.18M ± 0%  -0.64%  (p=0.000 n=20+20)
Compiler        4.60M ± 0%        4.58M ± 0%  -0.57%  (p=0.000 n=20+20)
SSA             11.5M ± 0%        11.4M ± 0%  -0.42%  (p=0.000 n=19+20)
Flate            235k ± 0%         233k ± 0%  -0.74%  (p=0.000 n=20+19)
GoParser         316k ± 0%         313k ± 0%  -0.69%  (p=0.000 n=20+20)
Reflect          953k ± 0%         946k ± 0%  -0.81%  (p=0.000 n=20+20)
Tar              391k ± 0%         388k ± 0%  -0.61%  (p=0.000 n=20+19)
XML              413k ± 0%         411k ± 0%  -0.56%  (p=0.000 n=20+20)

Change-Id: I7378174e3550b47df4368b24cf24c8ce1b85c906
Reviewed-on: https://go-review.googlesource.com/104656Reviewed-by: Daniel Martí <mvdan@mvdan.cc>