cmd/compile: move Node.Class to flags

cmd/compile: move Node.Class to flags

Put it at position zero, since it is fairly hot.

This shrinks gc.Node into a smaller size class on 64 bit systems.

name        old time/op       new time/op       delta
Template          193ms ± 5%        192ms ± 3%    ~     (p=0.353 n=94+93)
Unicode          86.1ms ± 5%       85.0ms ± 4%  -1.23%  (p=0.000 n=95+98)
GoTypes           546ms ± 3%        544ms ± 4%  -0.40%  (p=0.007 n=94+97)
Compiler          2.56s ± 3%        2.54s ± 3%  -0.67%  (p=0.000 n=99+97)
SSA               5.13s ± 2%        5.10s ± 3%  -0.55%  (p=0.000 n=94+98)
Flate             122ms ± 6%        121ms ± 4%  -0.75%  (p=0.002 n=97+95)
GoParser          144ms ± 5%        144ms ± 4%    ~     (p=0.298 n=98+97)
Reflect           348ms ± 4%        349ms ± 4%    ~     (p=0.350 n=98+97)
Tar               105ms ± 5%        104ms ± 5%    ~     (p=0.154 n=96+98)
XML               200ms ± 5%        198ms ± 4%  -0.71%  (p=0.015 n=97+98)
[Geo mean]        330ms             328ms       -0.52%

name        old user-time/op  new user-time/op  delta
Template          229ms ±11%        224ms ± 7%  -2.16%  (p=0.001 n=100+87)
Unicode           109ms ± 5%        109ms ± 6%    ~     (p=0.897 n=96+91)
GoTypes           712ms ± 4%        709ms ± 4%    ~     (p=0.085 n=96+98)
Compiler          3.41s ± 3%        3.36s ± 3%  -1.43%  (p=0.000 n=98+98)
SSA               7.46s ± 3%        7.31s ± 3%  -2.02%  (p=0.000 n=100+99)
Flate             145ms ± 6%        143ms ± 6%  -1.11%  (p=0.001 n=99+97)
GoParser          177ms ± 5%        176ms ± 5%  -0.78%  (p=0.018 n=95+95)
Reflect           432ms ± 7%        435ms ± 9%    ~     (p=0.296 n=100+100)
Tar               121ms ± 7%        121ms ± 5%    ~     (p=0.072 n=100+95)
XML               241ms ± 4%        239ms ± 5%    ~     (p=0.085 n=97+99)
[Geo mean]        413ms             410ms       -0.73%

name        old alloc/op      new alloc/op      delta
Template         38.4MB ± 0%       37.7MB ± 0%  -1.85%  (p=0.008 n=5+5)
Unicode          30.1MB ± 0%       28.8MB ± 0%  -4.09%  (p=0.008 n=5+5)
GoTypes           112MB ± 0%        110MB ± 0%  -1.69%  (p=0.008 n=5+5)
Compiler          470MB ± 0%        461MB ± 0%  -1.91%  (p=0.008 n=5+5)
SSA              1.13GB ± 0%       1.11GB ± 0%  -1.70%  (p=0.008 n=5+5)
Flate            25.0MB ± 0%       24.6MB ± 0%  -1.67%  (p=0.008 n=5+5)
GoParser         31.6MB ± 0%       31.1MB ± 0%  -1.66%  (p=0.008 n=5+5)
Reflect          77.1MB ± 0%       75.8MB ± 0%  -1.69%  (p=0.008 n=5+5)
Tar              26.3MB ± 0%       25.7MB ± 0%  -2.06%  (p=0.008 n=5+5)
XML              41.9MB ± 0%       41.1MB ± 0%  -1.93%  (p=0.008 n=5+5)
[Geo mean]       73.5MB            72.0MB       -2.03%

name        old allocs/op     new allocs/op     delta
Template           383k ± 0%         383k ± 0%    ~     (p=0.690 n=5+5)
Unicode            343k ± 0%         343k ± 0%    ~     (p=0.841 n=5+5)
GoTypes           1.16M ± 0%        1.16M ± 0%    ~     (p=0.310 n=5+5)
Compiler          4.43M ± 0%        4.42M ± 0%  -0.17%  (p=0.008 n=5+5)
SSA               9.85M ± 0%        9.85M ± 0%    ~     (p=0.310 n=5+5)
Flate              236k ± 0%         236k ± 1%    ~     (p=0.841 n=5+5)
GoParser           320k ± 0%         320k ± 0%    ~     (p=0.421 n=5+5)
Reflect            988k ± 0%         987k ± 0%    ~     (p=0.690 n=5+5)
Tar                252k ± 0%         251k ± 0%    ~     (p=0.095 n=5+5)
XML                399k ± 0%         399k ± 0%    ~     (p=1.000 n=5+5)
[Geo mean]         741k              740k       -0.07%

Change-Id: I9e952b58a98e30a12494304db9ce50d0a85e459c
Reviewed-on: https://go-review.googlesource.com/41797
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Marvin Stenger <marvin.stenger94@gmail.com>