Help optimize out bounds checks in median_of_medians

[kornelski]

LLVM can't see that assert!((v / 200) <= (v / 2)) is always true, so it couldn't remove bounds in median_of_ninthers.

.unwrap() outside of min_index() was losing bounds information. I've even tried assume(index < slice.len()); return Some(index), but it didn't help. Only moving the unwrap() to inside the function seems to keep the range information (and doesn't need assume).

Part of #144326

[rustbot]

r? @tgross35

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[tgross35]

Is this something that could get a codegen test to make sure we don't have a jump to a panic? Seems easy to accidentally regress.

[kornelski]

A codegen test would be valuable, but these functions are inlined into other functions that still need bounds checks fixed, so I can't realistically codegen tests until more of this work is done.