add load64

Much like store64, load64 really wants to use ld2.4s but that needs a
way to allocate adjacent registers.  So, just like store64, do it
manually, this time with uzp (unzip).

Change-Id: Ie10cc8d2df57390d1c6709bd7485bb5158375078
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/340519
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
3 files changed