Make load4 and store4 part of SkNx properly.

Every type now nominally has Load4() and Store4() methods.
The ones that we use are implemented.

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=3046
CQ_INCLUDE_TRYBOTS=master.client.skia:Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-SKNX_NO_SIMD-Trybot

Change-Id: I7984f0c2063ef8acbc322bd2e968f8f7eaa0d8fd
Reviewed-on: https://skia-review.googlesource.com/3046
Reviewed-by: Matt Sarett <msarett@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
6 files changed