[mlir] Rewrites for I2 to I8 signed and unsigned extension (#121298)

Adds rewrites for i2 to i8 signed and unsigned extension, similar to the
ones that already exist for i4 to i8 conversion.

I use this for i6 quantized models, and this gives me roughly a 2x
speedup for an i6 4096x4096 dequantization-matmul on an AMD 5950x.

I didn't add the rewrite for i8 to i2 truncation because I currently
don't use it, but if this is needed, I can add it as well.

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Co-authored-by: Andrzej Warzyński <andrzej.warzynski@gmail.com>

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