cmd/asm: fix the error of checking the post-index offset of VLD[1-4]R instructions of arm64

The post-index offset of VLD[1-4]R instructions is decided by the
"size" field not "Q" field, the current assembler uses "Q" fileld
to check the correctness of post-index offset which is not correct.
This patch fixes it.

Fixes #40725

Change-Id: If1cde7f21c6b3ee0e491649eb567700bd1475c84
Reviewed-on: https://go-review.googlesource.com/c/go/+/249757


Reviewed-by: Cherry Zhang <cherryyz@google.com>