looprotate finds loop headers and arranges for them to be placed
after the body of the loop. This eliminates a jump from the body.

However, if the loop header is a series of contiguously laid out blocks,
the rotation introduces a new jump in that series.
This CL expands the "loop header" to move to be the entire
run of contiguously laid out blocks in the same loop.

This shrinks object files a little, and actually speeds up
the compiler noticeably. Numbers below.

Fannkuch performance seems to vary a lot by machine. On my laptop:

name          old time/op  new time/op  delta
Fannkuch11-8   2.89s ± 2%   2.85s ± 3%  -1.22%  (p=0.000 n=50+50)

This has a significant affect on the append benchmarks in #14758:

name   old time/op    new time/op    delta
Foo-8     312ns ± 3%     276ns ± 2%  -11.37%  (p=0.000 n=30+29)
Bar-8     565ns ± 2%     456ns ± 2%  -19.27%  (p=0.000 n=27+28)

Updates #18977
Fixes #20355

name        old time/op       new time/op       delta
Template          205ms ± 5%        204ms ± 8%    ~     (p=0.903 n=92+99)
Unicode          85.3ms ± 4%       85.1ms ± 3%    ~     (p=0.191 n=92+94)
GoTypes           512ms ± 4%        507ms ± 4%  -0.93%  (p=0.000 n=95+97)
Compiler          2.38s ± 3%        2.35s ± 3%  -1.27%  (p=0.000 n=98+95)
SSA               4.67s ± 3%        4.64s ± 3%  -0.62%  (p=0.000 n=95+96)
Flate             117ms ± 3%        117ms ± 3%    ~     (p=0.099 n=84+86)
GoParser          139ms ± 4%        137ms ± 4%  -0.90%  (p=0.000 n=97+98)
Reflect           329ms ± 5%        326ms ± 6%  -0.97%  (p=0.002 n=99+98)
Tar               102ms ± 6%        101ms ± 5%  -0.97%  (p=0.006 n=97+97)
XML               198ms ±10%        196ms ±13%    ~     (p=0.087 n=100+100)
[Geo mean]        318ms             316ms       -0.72%

name        old user-time/op  new user-time/op  delta
Template          250ms ± 7%        250ms ± 7%    ~     (p=0.850 n=94+92)
Unicode           107ms ± 8%        106ms ± 5%  -0.76%  (p=0.005 n=98+91)
GoTypes           665ms ± 5%        659ms ± 5%  -0.85%  (p=0.003 n=93+98)
Compiler          3.15s ± 3%        3.10s ± 3%  -1.60%  (p=0.000 n=99+98)
SSA               6.82s ± 3%        6.72s ± 4%  -1.55%  (p=0.000 n=94+98)
Flate             138ms ± 8%        138ms ± 6%    ~     (p=0.369 n=94+92)
GoParser          170ms ± 5%        168ms ± 6%  -1.13%  (p=0.002 n=96+98)
Reflect           412ms ± 8%        416ms ± 8%    ~     (p=0.169 n=100+100)
Tar               123ms ±18%        123ms ±14%    ~     (p=0.896 n=100+100)
XML               236ms ± 9%        234ms ±11%    ~     (p=0.124 n=100+100)
[Geo mean]        401ms             398ms       -0.63%

name        old alloc/op      new alloc/op      delta
Template         38.8MB ± 0%       38.8MB ± 0%    ~     (p=0.222 n=5+5)
Unicode          28.7MB ± 0%       28.7MB ± 0%    ~     (p=0.421 n=5+5)
GoTypes           109MB ± 0%        109MB ± 0%    ~     (p=0.056 n=5+5)
Compiler          457MB ± 0%        457MB ± 0%  +0.07%  (p=0.008 n=5+5)
SSA              1.10GB ± 0%       1.10GB ± 0%  +0.05%  (p=0.008 n=5+5)
Flate            24.5MB ± 0%       24.5MB ± 0%    ~     (p=0.222 n=5+5)
GoParser         30.9MB ± 0%       31.0MB ± 0%  +0.21%  (p=0.016 n=5+5)
Reflect          73.4MB ± 0%       73.4MB ± 0%    ~     (p=0.421 n=5+5)
Tar              25.5MB ± 0%       25.5MB ± 0%    ~     (p=0.548 n=5+5)
XML              40.9MB ± 0%       40.9MB ± 0%    ~     (p=0.151 n=5+5)
[Geo mean]       71.6MB            71.6MB       +0.07%

name        old allocs/op     new allocs/op     delta
Template           394k ± 0%         394k ± 0%    ~     (p=1.000 n=5+5)
Unicode            344k ± 0%         343k ± 0%    ~     (p=0.310 n=5+5)
GoTypes           1.16M ± 0%        1.16M ± 0%    ~     (p=1.000 n=5+5)
Compiler          4.42M ± 0%        4.42M ± 0%    ~     (p=1.000 n=5+5)
SSA               9.80M ± 0%        9.80M ± 0%    ~     (p=0.095 n=5+5)
Flate              237k ± 1%         238k ± 1%    ~     (p=0.310 n=5+5)
GoParser           320k ± 0%         322k ± 1%  +0.50%  (p=0.032 n=5+5)
Reflect            958k ± 0%         957k ± 0%    ~     (p=0.548 n=5+5)
Tar                252k ± 1%         252k ± 0%    ~     (p=1.000 n=5+5)
XML                400k ± 0%         400k ± 0%    ~     (p=0.841 n=5+5)
[Geo mean]         741k              742k       +0.06%

name        old object-bytes  new object-bytes  delta
Template           386k ± 0%         386k ± 0%  -0.05%  (p=0.008 n=5+5)
Unicode            202k ± 0%         202k ± 0%  -0.01%  (p=0.008 n=5+5)
GoTypes           1.16M ± 0%        1.16M ± 0%  -0.06%  (p=0.008 n=5+5)
Compiler          3.91M ± 0%        3.91M ± 0%  -0.06%  (p=0.008 n=5+5)
SSA               7.91M ± 0%        7.92M ± 0%  +0.01%  (p=0.008 n=5+5)
Flate              228k ± 0%         227k ± 0%  -0.04%  (p=0.008 n=5+5)
GoParser           283k ± 0%         283k ± 0%  -0.06%  (p=0.008 n=5+5)
Reflect            952k ± 0%         951k ± 0%  -0.02%  (p=0.008 n=5+5)
Tar                187k ± 0%         187k ± 0%  -0.04%  (p=0.008 n=5+5)
XML                406k ± 0%         406k ± 0%  -0.05%  (p=0.008 n=5+5)
[Geo mean]         648k              648k       -0.04%

Change-Id: I8630c4291a0eb2f7e7927bc04d7cc0efef181094
Reviewed-on: https://go-review.googlesource.com/43491Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>