I16x8SConvertI32x4, I16x8UConvertI32x4,
I8x16SConvertI16x8, I8x16UConvertI16x8

Add packsswb/packssdw/packuswb/packusdw

Change-Id: Ibb661a20fa032d732fec20b3d48190f44d2d4bd4
Reviewed-on: https://chromium-review.googlesource.com/1027123Reviewed-by: Bill Budge <bbudge@chromium.org>
Commit-Queue: Jing Bao <jing.bao@intel.com>
Cr-Commit-Position: refs/heads/master@{#52870}