allow a fourth Val/Reg arg per op

This is a pure refactor, making room for a fourth argument per Op.

I'll use this to make store128 work more naturally, taking all
128 bits of its input at once, rather than spreading across two strided
64-bit stores as done today.  I've left this for a second CL because
this refactor on its own seemed error-prone enough.

Change-Id: I704e4f2e165b0bfd0276af921d97bf2cc01e8550
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/355659
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
3 files changed