For details see the discussion on the issue below.

RotateLeft functions can now be inlined because the don't panic
anymore for negative rotation counts.

name            old time/op  new time/op  delta
RotateLeft-8    6.72ns ± 2%  1.86ns ± 0%  -72.33%  (p=0.016 n=5+4)
RotateLeft8-8   4.41ns ± 2%  1.67ns ± 1%  -62.15%  (p=0.008 n=5+5)
RotateLeft16-8  4.46ns ± 6%  1.65ns ± 0%  -63.06%  (p=0.008 n=5+5)
RotateLeft32-8  4.50ns ± 5%  1.67ns ± 1%  -62.86%  (p=0.008 n=5+5)
RotateLeft64-8  4.54ns ± 1%  1.85ns ± 1%  -59.32%  (p=0.008 n=5+5)

https://perf.golang.org/search?q=upload:20170411.4

(Measured on 2.3 GHz Intel Core i7 running macOS 10.12.3.)

For #18616.

Change-Id: I0828d80d54ec24f8d44954a57b3d6aeedb69c686
Reviewed-on: https://go-review.googlesource.com/40394Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>