cmd/asm: complete the support for VDUP on arm64

"VMOV Vn.<T>[index], Vn" is equivalent to "VDUP Vn.<T>[index], Vn", and
the latter has a higher priority in the disassembler than the former.
But the assembler doesn't support to encode this combination of VDUP,
this leads to an inconsistency between assembler and disassembler.

For example, if we assemble "VMOV V20.S[0], V20" to hex then decode it,
we'll get "VDUP V20.S[0], V20".

  VMOV V20.S[0], V20 -> 9406045e -> VDUP V20.S[0], V20 -> error

But we cannot assemble this VDUP again.

Similar reason for "VDUP Rn, Vd.<T>". This CL completes the support for
VDUP.

This patch is a copy of CL 276092. Co-authored-by: JunchenLi
<junchen.li@arm.com>

Change-Id: I8f8d86cf1911d5b16bb40d189f1dc34b24416aaf
Reviewed-on: https://go-review.googlesource.com/c/go/+/302929


Trust: fannie zhang <Fannie.Zhang@arm.com>
Run-TryBot: fannie zhang <Fannie.Zhang@arm.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>