[mlir][sparse] Change the quick sort pivot selection.

Previously, we choose the value at (lo + hi)/2 as a pivot for partitioning the
data in [lo, hi). We now choose the median for the three values at lo, (lo +
hi)/2, and (hi-1) as a pivot to match the std::qsort implementation.

Reviewed By: aartbik

Differential Revision: https://reviews.llvm.org/D142679