cmd/compile: reuse []Flow

cmd/compile: reuse []Flow

Benchmarked using compilebench on a quiet
but rather old OS X laptop.

Benchmarks from others would be welcome,
since the numbers look too good to be true.

name      old time/op    new time/op    delta
Template     331ms ± 9%     303ms ± 4%   -8.25%  (p=0.000 n=24+24)
GoTypes      946ms ± 4%     888ms ± 3%   -6.17%  (p=0.000 n=24+25)
Compiler     3.20s ± 1%     3.10s ± 2%   -3.07%  (p=0.000 n=24+25)

name      old alloc/op   new alloc/op   delta
Template    72.5MB ± 0%    61.8MB ± 0%  -14.76%  (p=0.000 n=25+24)
GoTypes      224MB ± 0%     189MB ± 0%  -15.65%  (p=0.000 n=25+25)
Compiler     695MB ± 0%     561MB ± 0%  -19.26%  (p=0.000 n=25+25)

name      old allocs/op  new allocs/op  delta
Template      498k ± 0%      497k ± 0%   -0.21%  (p=0.000 n=25+23)
GoTypes      1.47M ± 0%     1.47M ± 0%   -0.25%  (p=0.000 n=25+25)
Compiler     4.09M ± 0%     4.08M ± 0%   -0.18%  (p=0.000 n=25+23)

Change-Id: I2394bc748128d721863453257fa5756c410f7898
Reviewed-on: https://go-review.googlesource.com/19771
Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>