cmd/compile: add restrictions on the shift amount range of arm64 various instructions

cmd/compile: add restrictions on the shift amount range of arm64 various instructions

For arm64 constant shift instructions, e.g., LSL(immediate), they use
only the low 6 bits. To conform the semantics of the hardware instructions,
this CL comments in ARM64OPS.go about restricted AuxInt ranges for the
various instructions involved.

Change-Id: I2b6560d6580e22ba7cbfa744a02b046dd5714b8a
Reviewed-on: https://go-review.googlesource.com/c/go/+/303569
Trust: fannie zhang <Fannie.Zhang@arm.com>
Reviewed-by: Keith Randall <khr@golang.org>