Flip scanning direction of stable sort

Memory pre-fetching prefers forward scanning vs backwards scanning, and the
code-gen is usually better. For the most sensitive types such as integers, these
are planned to be merged bidirectionally at once. So there is no benefit in
scanning backwards.

The largest perf gains are seen for full ascending and descending inputs, which
see 1.5x speedups. Random inputs benefit too, and some patterns can loose out,
but these losses are minimal.

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