This change fixes index error when encoding VMOV instruction which pattern
is vmov Vn.<T>[index], Vd.<T>[index]

Change-Id: I949166e6dfd63fb0a9365f183b6c50d452614f9d
Reviewed-on: https://go-review.googlesource.com/101335Reviewed-by: Cherry Zhang <cherryyz@google.com>