fix: remove loop_len from DictOxide - for loop unwind black_box

[dishmaker]

My suggestion from:

• Massive performance regression in deflate compression between rust 1.81 and 1.82 due to upstream rust compiler optimization issue #163

[oyvindln]

When testing this approach quickly it seemed like it made things very slightly slower than the current approach for whatever reason so will have to do a bit more testing. Might just have been a fluke though.

[oyvindln]

Seems to not have been a fluke and it does indeed perform worse than the current solution, though I didn't dig into the assmebly output to investigate why exactly.