The ShlPair function has two out of order instructions, this patch
reorder them and add some checks to prevent the dst register from
conflicting with the src register.

Change-Id: I2dd4b20a5c55fbbe75b126162302997acec5a6bb
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1936647
Auto-Submit: Zhao Jiazhong <zhaojiazhong-hf@loongson.cn>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65188}