cmd/compile: optimize RotateLeft8/16 on arm64

cmd/compile: optimize RotateLeft8/16 on arm64

This CL optimizes RotateLeft8/16 on arm64.

For 16 bits, we form a 32 bits register by duplicating two 16 bits
registers, then use RORW instruction to do the rotate shift.

For 8 bits, we just use LSR and LSL instead of RORW because the code is
simpler.

Benchmark          Old          ThisCL       delta
RotateLeft8-46     2.16 ns/op   1.73 ns/op   -19.70%
RotateLeft16-46    2.16 ns/op   1.54 ns/op   -28.53%

Change-Id: I09cde4383d12e31876a57f8cdfd3bb4f324fadb0
Reviewed-on: https://go-review.googlesource.com/c/go/+/420976
Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Run-TryBot: Keith Randall <khr@golang.org>