Improve predict vs no-predict heuristic.
Improve predict vs no-predict heuristic.

This improves compression density. For example, at quality 95 on 1000 PNGs:
bpp(before) = 2.447 and bpp(after) = 2.412

Change-Id: I19c343ba05cca48a6940293721066502a5c3d693

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