JIT to_half/from_half
JIT to_half/from_half

Tricky parts:
   - remembering that we're carrying these 8 half floats around
     in a ymm as an I32, not the more natural dense xmm;
   - remembering to fix up the middle 64-bit lanes after vpackusdw
     leaves things in an order you'd have to be Intel to love.

The tests from the previous CL cover this, and pass.

Change-Id: I5dfde54c1392388c82ee9c8274b83bc1d63b6577
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/303045
Reviewed-by: Herb Derby <9e12a3d2bbf73546e47cc9958f6ba8f3215b7eaf@google.com>
Commit-Queue: Mike Klein <14574f09dfa9b4e14759b88c3426a495a0e627b0@google.com>

google/skia,google/skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,google/skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,google/skia,google/skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia