Auto merge of #117179 - Voultapher:fix-useless-comp-in-partition-equal, r=Mark-Simulacrum

Avoid unnecessary comparison in partition_equal

The branchy Hoare partition `partition_equal` as part of `slice::sort_unstable` has a bug that makes it perform a comparison of the last element twice.

Measuring inputs with a Zipfian distribution with characterizing exponent s == 1.0, yields a ~0.05% reduction in the total number of comparisons performed.

Author: bors