We already prefetch in case of large inputs, do the same
for medium sized inputs as well. This is mostly neutral
for performance in most cases, so this also adds a new
bench with working size >> cache size to ensure that we
are seeing performance benefits of prefetch. Main benefits
are on AMD with hardware prefetchers turned off:

AMD prefetchers on:
name                           old time/op  new time/op  delta
BM_Calculate/0                 2.43ns ± 1%  2.43ns ± 1%     ~     (p=0.814 n=40+40)
BM_Calculate/1                 2.50ns ± 2%  2.50ns ± 2%     ~     (p=0.745 n=39+39)
BM_Calculate/100               9.17ns ± 1%  9.17ns ± 2%     ~     (p=0.747 n=40+40)
BM_Calculate/10000              474ns ± 1%   474ns ± 2%     ~     (p=0.749 n=40+40)
BM_Calculate/500000            22.8µs ± 1%  22.9µs ± 2%     ~     (p=0.298 n=39+40)
BM_Extend/0                    1.38ns ± 1%  1.38ns ± 1%     ~     (p=0.651 n=40+40)
BM_Extend/1                    1.53ns ± 2%  1.53ns ± 1%     ~     (p=0.957 n=40+39)
BM_Extend/100                  9.48ns ± 1%  9.48ns ± 2%     ~     (p=1.000 n=40+40)
BM_Extend/10000                 474ns ± 2%   474ns ± 1%     ~     (p=0.928 n=40+40)
BM_Extend/500000               22.8µs ± 1%  22.9µs ± 2%     ~     (p=0.331 n=40+40)
BM_Extend/100000000            4.79ms ± 1%  4.79ms ± 1%     ~     (p=0.753 n=38+38)
BM_ExtendCacheMiss/10          25.5ms ± 2%  25.5ms ± 2%     ~     (p=0.988 n=38+40)
BM_ExtendCacheMiss/100         23.1ms ± 2%  23.1ms ± 2%     ~     (p=0.792 n=40+40)
BM_ExtendCacheMiss/1000        37.2ms ± 1%  28.6ms ± 2%  -23.00%  (p=0.000 n=38+40)
BM_ExtendCacheMiss/100000      7.77ms ± 2%  7.74ms ± 2%   -0.45%  (p=0.006 n=40+40)

AMD prefetchers off:
name                           old time/op  new time/op  delta
BM_Calculate/0                 2.43ns ± 2%  2.43ns ± 2%     ~     (p=0.351 n=40+39)
BM_Calculate/1                 2.51ns ± 2%  2.51ns ± 1%     ~     (p=0.535 n=40+40)
BM_Calculate/100               9.18ns ± 2%  9.15ns ± 2%     ~     (p=0.120 n=38+39)
BM_Calculate/10000              475ns ± 2%   475ns ± 2%     ~     (p=0.852 n=40+40)
BM_Calculate/500000            22.9µs ± 2%  22.8µs ± 2%     ~     (p=0.396 n=40+40)
BM_Extend/0                    1.38ns ± 2%  1.38ns ± 2%     ~     (p=0.466 n=40+40)
BM_Extend/1                    1.53ns ± 2%  1.53ns ± 2%     ~     (p=0.914 n=40+39)
BM_Extend/100                  9.49ns ± 2%  9.49ns ± 2%     ~     (p=0.802 n=40+40)
BM_Extend/10000                 475ns ± 2%   474ns ± 1%     ~     (p=0.589 n=40+40)
BM_Extend/500000               22.8µs ± 2%  22.8µs ± 2%     ~     (p=0.872 n=39+40)
BM_Extend/100000000            10.0ms ± 3%  10.0ms ± 4%     ~     (p=0.355 n=40+40)
BM_ExtendCacheMiss/10           196ms ± 2%   196ms ± 2%     ~     (p=0.698 n=40+40)
BM_ExtendCacheMiss/100          129ms ± 1%   129ms ± 1%     ~     (p=0.602 n=36+37)
BM_ExtendCacheMiss/1000        88.6ms ± 1%  57.2ms ± 1%  -35.49%  (p=0.000 n=36+38)
BM_ExtendCacheMiss/100000      14.9ms ± 1%  14.9ms ± 1%     ~     (p=0.888 n=39+40)

Intel skylake:
BM_Calculate/0                 2.49ns ± 2%  2.44ns ± 4%  -2.15%  (p=0.001 n=31+34)
BM_Calculate/1                 3.04ns ± 2%  2.98ns ± 9%  -1.95%  (p=0.003 n=31+35)
BM_Calculate/100               8.64ns ± 3%  8.53ns ± 5%    ~     (p=0.065 n=31+35)
BM_Calculate/10000              290ns ± 3%   285ns ± 7%  -1.80%  (p=0.004 n=28+34)
BM_Calculate/500000            11.8µs ± 2%  11.6µs ± 8%  -1.59%  (p=0.003 n=26+34)
BM_Extend/0                    1.56ns ± 1%  1.52ns ± 3%  -2.44%  (p=0.000 n=26+35)
BM_Extend/1                    1.88ns ± 3%  1.83ns ± 6%  -2.17%  (p=0.001 n=27+35)
BM_Extend/100                  9.31ns ± 3%  9.13ns ± 7%  -1.92%  (p=0.000 n=33+38)
BM_Extend/10000                 290ns ± 3%   283ns ± 3%  -2.45%  (p=0.000 n=32+38)
BM_Extend/500000               11.8µs ± 2%  11.5µs ± 8%  -1.80%  (p=0.001 n=35+37)
BM_Extend/100000000            6.39ms ±10%  6.11ms ± 8%  -4.34%  (p=0.000 n=40+40)
BM_ExtendCacheMiss/10          36.2ms ± 7%  35.8ms ±14%    ~     (p=0.281 n=33+37)
BM_ExtendCacheMiss/100         26.9ms ±15%  25.9ms ±12%  -3.93%  (p=0.000 n=40+40)
BM_ExtendCacheMiss/1000        23.8ms ± 5%  23.4ms ± 5%  -1.68%  (p=0.001 n=39+40)
BM_ExtendCacheMiss/100000      10.1ms ± 5%  10.0ms ± 4%    ~     (p=0.051 n=39+39)

PiperOrigin-RevId: 495119444
Change-Id: I67bcf3b0282b5e1c43122de2837a24c16b8aded7