reflectdata: unroll a loop in array equal function generation

reflectdata: unroll a loop in array equal function generation

As josharian mentioned, a compare function could benefit from
unrolling a loop for arrays. This commit introduces such
functionality.

name                     old time/op  new time/op  delta
EqArrayOfStrings5-12     12.5ns ± 1%   8.4ns ± 1%  -33.05%  (p=0.008 n=5+5)
EqArrayOfStrings64-12    71.7ns ± 1%  64.1ns ± 1%  -10.57%  (p=0.008 n=5+5)
EqArrayOfStrings1024-12  1.12µs ± 1%  1.01µs ± 0%   -9.77%  (p=0.008 n=5+5)
[Geo mean]                100ns         81ns       -18.56%

name                    old time/op  new time/op  delta
EqArrayOfFloats5-12     4.50ns ± 2%  3.32ns ± 1%  -26.09%  (p=0.008 n=5+5)
EqArrayOfFloats64-12    41.3ns ± 1%  35.7ns ± 0%  -13.63%  (p=0.016 n=5+4)
EqArrayOfFloats1024-12   619ns ± 1%   557ns ± 1%   -9.95%  (p=0.008 n=5+5)
[Geo mean]              48.6ns       40.4ns       -16.85%

Change-Id: If1b69c5cf3fb246bb0275a292118b0b93ad9c9a8
Reviewed-on: https://go-review.googlesource.com/c/go/+/368614
Reviewed-by: Keith Randall <khr@golang.org>
Trust: Emmanuel Odeke <emmanuel@orijtech.com>
Run-TryBot: Emmanuel Odeke <emmanuel@orijtech.com>
TryBot-Result: Gopher Robot <gobot@golang.org>