add mad_unorm8 instruction

At some point adding more and more complex instructions reduces
to the absurdity of SolveTheWholeProblem-The-Instruction, but
I think this one will come up often enough to still make sense.
mad() makes sense for unorm8 just about everywhere mad() makes
sense for f32.

This instruction won't matter to a JIT, but helps the interpreter.

Change-Id: Iace92296cffbb6fbc3acd1f853cb01c51792f796
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/218716
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
1 file changed