(platforms w/o corresponding assembly kernels)

For short vector adds there's some erradic slow-down, but overall
these routines have become significantly faster. This only matters
for platforms w/o native (assembly) versions of these kernels, so
we are not concerned about the minor slow-down for short vectors.

This code was already reviewed under Mercurial (golang.org/cl/172810043)
but wasn't submitted before the switch to git.

Benchmarks run on 2.3GHz Intel Core i7, running OS X 10.9.5,
with the respective AddVV and AddVW assembly routines disabled.

benchmark              old ns/op     new ns/op     delta
BenchmarkAddVV_1       6.59          7.09          +7.59%
BenchmarkAddVV_2       10.3          10.1          -1.94%
BenchmarkAddVV_3       10.9          12.6          +15.60%
BenchmarkAddVV_4       13.9          15.6          +12.23%
BenchmarkAddVV_5       16.8          17.3          +2.98%
BenchmarkAddVV_1e1     29.5          29.9          +1.36%
BenchmarkAddVV_1e2     246           232           -5.69%
BenchmarkAddVV_1e3     2374          2185          -7.96%
BenchmarkAddVV_1e4     58942         22292         -62.18%
BenchmarkAddVV_1e5     668622        225279        -66.31%
BenchmarkAddVW_1       6.81          5.58          -18.06%
BenchmarkAddVW_2       7.69          6.86          -10.79%
BenchmarkAddVW_3       9.56          8.32          -12.97%
BenchmarkAddVW_4       12.1          9.53          -21.24%
BenchmarkAddVW_5       13.2          10.9          -17.42%
BenchmarkAddVW_1e1     23.4          18.0          -23.08%
BenchmarkAddVW_1e2     175           141           -19.43%
BenchmarkAddVW_1e3     1568          1266          -19.26%
BenchmarkAddVW_1e4     15425         12596         -18.34%
BenchmarkAddVW_1e5     156737        133539        -14.80%
BenchmarkFibo          381678466     132958666     -65.16%

benchmark              old MB/s     new MB/s     speedup
BenchmarkAddVV_1       9715.25      9028.30      0.93x
BenchmarkAddVV_2       12461.72     12622.60     1.01x
BenchmarkAddVV_3       17549.64     15243.82     0.87x
BenchmarkAddVV_4       18392.54     16398.29     0.89x
BenchmarkAddVV_5       18995.23     18496.57     0.97x
BenchmarkAddVV_1e1     21708.98     21438.28     0.99x
BenchmarkAddVV_1e2     25956.53     27506.88     1.06x
BenchmarkAddVV_1e3     26947.93     29286.66     1.09x
BenchmarkAddVV_1e4     10857.96     28709.46     2.64x
BenchmarkAddVV_1e5     9571.91      28409.21     2.97x
BenchmarkAddVW_1       1175.28      1433.98      1.22x
BenchmarkAddVW_2       2080.01      2332.54      1.12x
BenchmarkAddVW_3       2509.28      2883.97      1.15x
BenchmarkAddVW_4       2646.09      3356.83      1.27x
BenchmarkAddVW_5       3020.69      3671.07      1.22x
BenchmarkAddVW_1e1     3425.76      4441.40      1.30x
BenchmarkAddVW_1e2     4553.17      5642.96      1.24x
BenchmarkAddVW_1e3     5100.14      6318.72      1.24x
BenchmarkAddVW_1e4     5186.15      6350.96      1.22x
BenchmarkAddVW_1e5     5104.07      5990.74      1.17x

Change-Id: I7a62023b1105248a0e85e5b9819d3fd4266123d4
Reviewed-on: https://go-review.googlesource.com/2480Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>