[MachineOutliner] Sort Outlining Functions by Priority

[xuanzhang816]

We changed the order in which functions are outlined in Machine Outliner.

In MachineOutliner::outline(), functions are sorted by their benefits first and then outlined one at a time greedily. Our investigation shows that sorting the functions by a different value, which we call priority, could be more beneficial. For clang, sorting the functions by priority reduces the binary size by 1.8% compared to the baseline; if we additionally consider leaf descendants to include more candidates, the binary size is reduced by 2.8% compared to the baseline.

The formula for priority is found via a black-box Bayesian optimization toolbox, and we have shown that using this formula for sorting consistently reduces uncompressed mobile app size within our company.

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