add vgatherdps

A complicated instruction to say the least!

A "fun" wrinkle is that all the ymm registers must be unique!
(And the mask register is cleared by the instruction...)

Still kind of TODO is what that 0b100 r/m in the mod_rm() means.  Every
variant of the instruction I've assembled seems to have it set to 0b100
(e.g. 0x0c or 0x04) but I'd feel better if I knew what it meant.

Change-Id: Ia4ff5f8175bff545e2d10bb2d1b14f49073445a3
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/264116
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Herb Derby <herb@google.com>
3 files changed