clusterizer: Replace Hoare partition with branchless Lomuto partition
clusterizer: Replace Hoare partition with branchless Lomuto partition

The branchless variant is shorter and faster for our workloads, and also
easy to reason about. Note that this isn't exactly equivalent to a
Lomuto partition because we always swap the element but sometimes swap
it into the >= half, which means that the resulting partitioning isn't
stable.

zeux/meshoptimizer,zeux/meshoptimizer,zeux/meshoptimizer