Make `edata_cmp_summary_comp` 30% faster

`edata_cmp_summary_comp` is one of the very hottest functions, taking up
3% of all time spent inside Jemalloc. I noticed that all existing
callsites rely only on the sign of the value returned by this function,
so I came up with this equivalent branchless implementation which
preserves this property. After empirical measurement, I have found that
this implementation is 30% faster, therefore representing a 1% speed-up
to the allocator as a whole.

At @interwq's suggestion, I've applied the same optimization to
`edata_esnead_comp` in case this function becomes hotter in the future.

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