cmd/compile: optimize shift ops on arm64 when the shift value is v&63

cmd/compile: optimize shift ops on arm64 when the shift value is v&63

For the following code case:

  var x uint64
  x >> (shift & 63)

We can directly genereta `x >> shift` on arm64, since the hardware will
only use the bottom 6 bits of the shift amount.

Benchmark               old time/op  new time/op    delta
ShiftArithmeticRight-8  0.40ns       0.31ns        -21.7%

Change-Id: Id58c8a5b2f6dd5c30c3876f4a36e11b4d81e2dc9
Reviewed-on: https://go-review.googlesource.com/c/go/+/425294
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>