cmd/asm/internal/arch: adds the missing type check for arm64 SXTB extension

Operands of memory type do not support SXTB extension. This CL adds this
missing check.

Change-Id: I1fa438dd314fc8aeb889637079cc67b538e83a89
Reviewed-on: https://go-review.googlesource.com/c/go/+/342769


Reviewed-by: eric fang <eric.fang@arm.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: eric fang <eric.fang@arm.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Michael Knyszek <mknyszek@google.com>