Color dodge and burn with SkPMFloat.

Both 25-35% faster with SSE.
With NEON, Burn measures as a ~10% regression, Dodge a huge 2.9x improvement.

The Burn regression is somewhat artificial: we're drawing random colored rects onto an opaque white dst, so we're heavily biased toward the (d==da) fast path in the serial code.  In the vector code there's no short-circuiting and we always pay a fixed cost for ColorBurn regardless of src or dst content.

Dodge's fast paths, in contrast, only trigger when (s==sa) or (d==0), neither of which happens any more than randomly in our benchmark.  I don't think (d==0) should happen at all.  Similarly, the (s==0) Burn fast path is really only going to happen as often as SkRandom allows.

In practice, the existing Burn benchmark is hitting its fast path 100% of the time.  So I actually feel really great that this only dings the benchmark by 10%.

Chrome's still guarded by SK_SUPPORT_LEGACY_XFERMODES, which I'll lift after finishing the last xfermode, SoftLight.

BUG=skia:

Review URL: https://codereview.chromium.org/1214443002

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