Implement i8x16.popcnt, double precision conversions,
v64x2.alltrue and i64x2.ne.

Port: 00babf07
Port: 66964c51
Port: 1b81ffb1

Change-Id: I26f62f31fd362b06f7bcbf1988a8e2ff12d12504
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2706354
Auto-Submit: Liu yu <liuyu@loongson.cn>
Reviewed-by: Zhao Jiazhong <zhaojiazhong-hf@loongson.cn>
Commit-Queue: Zhao Jiazhong <zhaojiazhong-hf@loongson.cn>
Cr-Commit-Position: refs/heads/master@{#72854}