cmd/compile: stack-allocate values worklist in schedule

cmd/compile: stack-allocate values worklist in schedule

Compiler instrumentation shows that the cap of the stores slice in the
storeOrder function is almost always 64 or less. Since the slice does
not escape, pre-allocating on the stack a 64-elements one greatly
reduces the number of allocations performed by the function.

name      old time/op       new time/op       delta
Template        289ms ± 5%        283ms ± 3%  -1.99%  (p=0.000 n=19+20)
Unicode         140ms ± 6%        136ms ± 6%  -2.61%  (p=0.021 n=19+20)
GoTypes         915ms ± 2%        895ms ± 2%  -2.24%  (p=0.000 n=19+20)
Compiler        4.15s ± 1%        4.04s ± 2%  -2.73%  (p=0.000 n=20+20)
SSA             10.0s ± 1%         9.8s ± 2%  -2.13%  (p=0.000 n=20+20)
Flate           189ms ± 6%        186ms ± 4%  -1.75%  (p=0.028 n=19+20)
GoParser        229ms ± 5%        224ms ± 4%  -2.25%  (p=0.001 n=20+19)
Reflect         584ms ± 2%        573ms ± 3%  -1.83%  (p=0.000 n=18+20)
Tar             265ms ± 3%        261ms ± 3%  -1.33%  (p=0.021 n=20+20)
XML             328ms ± 2%        321ms ± 2%  -2.11%  (p=0.000 n=20+20)

name      old user-time/op  new user-time/op  delta
Template        408ms ± 4%        400ms ± 4%  -1.98%  (p=0.006 n=19+20)
Unicode         216ms ± 9%        216ms ± 7%    ~     (p=0.883 n=20+20)
GoTypes         1.25s ± 1%        1.23s ± 3%  -1.32%  (p=0.002 n=19+20)
Compiler        5.77s ± 1%        5.69s ± 2%  -1.47%  (p=0.000 n=18+19)
SSA             14.6s ± 5%        14.1s ± 4%  -3.45%  (p=0.000 n=20+20)
Flate           252ms ± 7%        251ms ± 7%    ~     (p=0.659 n=20+20)
GoParser        314ms ± 5%        310ms ± 5%    ~     (p=0.165 n=20+20)
Reflect         780ms ± 2%        769ms ± 3%  -1.34%  (p=0.004 n=19+18)
Tar             365ms ± 7%        367ms ± 5%    ~     (p=0.841 n=20+20)
XML             439ms ± 4%        432ms ± 4%  -1.45%  (p=0.043 n=20+20)

name      old alloc/op      new alloc/op      delta
Template       38.9MB ± 0%       38.8MB ± 0%  -0.26%  (p=0.000 n=19+20)
Unicode        29.0MB ± 0%       29.0MB ± 0%  -0.02%  (p=0.001 n=20+19)
GoTypes         115MB ± 0%        115MB ± 0%  -0.31%  (p=0.000 n=20+20)
Compiler        492MB ± 0%        490MB ± 0%  -0.41%  (p=0.000 n=20+19)
SSA            1.40GB ± 0%       1.39GB ± 0%  -0.48%  (p=0.000 n=20+20)
Flate          24.9MB ± 0%       24.9MB ± 0%  -0.24%  (p=0.000 n=20+20)
GoParser       30.9MB ± 0%       30.8MB ± 0%  -0.39%  (p=0.000 n=20+20)
Reflect        77.1MB ± 0%       76.8MB ± 0%  -0.32%  (p=0.000 n=17+20)
Tar            39.1MB ± 0%       39.0MB ± 0%  -0.23%  (p=0.000 n=20+20)
XML            44.7MB ± 0%       44.6MB ± 0%  -0.30%  (p=0.000 n=20+18)

name      old allocs/op     new allocs/op     delta
Template         385k ± 0%         382k ± 0%  -0.99%  (p=0.000 n=20+19)
Unicode          336k ± 0%         336k ± 0%  -0.08%  (p=0.000 n=19+17)
GoTypes         1.20M ± 0%        1.18M ± 0%  -1.11%  (p=0.000 n=20+18)
Compiler        4.66M ± 0%        4.59M ± 0%  -1.42%  (p=0.000 n=19+20)
SSA             11.6M ± 0%        11.5M ± 0%  -1.49%  (p=0.000 n=20+20)
Flate            237k ± 0%         235k ± 0%  -1.00%  (p=0.000 n=20+19)
GoParser         319k ± 0%         315k ± 0%  -1.12%  (p=0.000 n=20+20)
Reflect          960k ± 0%         952k ± 0%  -0.92%  (p=0.000 n=18+20)
Tar              394k ± 0%         390k ± 0%  -0.87%  (p=0.000 n=20+20)
XML              418k ± 0%         413k ± 0%  -1.18%  (p=0.000 n=20+20)

Change-Id: I01b9f45b161379967d7a52e23f39ac30dd90edb0
Reviewed-on: https://go-review.googlesource.com/104415
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>