Issue 6754 reports that Go bzip2 Decode function is much slower
(about 2.5x in go1.5) than the Python equivalent (which is
actually just a wrapper around the usual C library) on random data.

Profiling the code shows that half a dozen of CMP instructions in a
tight loop are responsibile for most of the execution time.

This patch reduces the number of branches of the loop, greatly
improving performance on random data and speeding up decoding of
real data.

name            old time/op    new time/op    delta
DecodeDigits-4    9.28ms ± 1%    8.05ms ± 1%  -13.18%  (p=0.000 n=15+14)
DecodeTwain-4     28.9ms ± 2%    26.4ms ± 1%   -8.57%  (p=0.000 n=15+14)
DecodeRand-4      3.94ms ± 1%    3.06ms ± 1%  -22.45%  (p=0.000 n=15+14)

name            old speed      new speed      delta
DecodeDigits-4  4.65MB/s ± 1%  5.36MB/s ± 1%  +15.21%  (p=0.000 n=13+14)
DecodeTwain-4   4.32MB/s ± 2%  4.72MB/s ± 1%   +9.36%  (p=0.000 n=15+14)
DecodeRand-4    4.27MB/s ± 1%  5.51MB/s ± 1%  +28.86%  (p=0.000 n=15+14)

I've run some benchmark comparing Go bzip2 implementation with the
usual Linux bzip2 command (which is written in C). On my machine
this patch brings go1.5
  from ~2.26x to ~1.50x of bzip2 time (on 64MB  random data)
  from ~1.70x to ~1.50x of bzip2 time (on 100MB english text)
  from ~2.00x to ~1.88x of bzip2 time (on 64MB  /dev/zero data)

Fixes #6754

Change-Id: I3cb12d2c0c2243c1617edef1edc88f05f91d26d1
Reviewed-on: https://go-review.googlesource.com/13853Reviewed-by: Nigel Tao <nigeltao@golang.org>