[mlir] Add polynomial approximation for math::Log2

```
name                     old cpu/op  new cpu/op  delta
BM_mlir_Log2_f32/10       134ns ±15%    45ns ± 4%  -66.39%  (p=0.000 n=20+17)
BM_mlir_Log2_f32/100     1.03µs ±16%  0.12µs ±10%  -88.78%  (p=0.000 n=20+18)
BM_mlir_Log2_f32/1k      10.3µs ±16%   0.7µs ± 5%  -93.24%  (p=0.000 n=20+17)
BM_mlir_Log2_f32/10k      104µs ±15%     7µs ±14%  -93.25%  (p=0.000 n=20+20)
BM_eigen_s_Log2_f32/10   95.3ns ±17%  90.9ns ± 6%     ~     (p=0.228 n=20+18)
BM_eigen_s_Log2_f32/100   907ns ± 3%   911ns ± 6%     ~     (p=0.539 n=16+20)
BM_eigen_s_Log2_f32/1k   9.88µs ± 4%  9.85µs ± 3%     ~     (p=0.790 n=16+17)
BM_eigen_s_Log2_f32/10k   105µs ±10%   110µs ±16%     ~     (p=0.459 n=16+20)
BM_eigen_v_Log2_f32/10   32.5ns ±31%  33.9ns ±14%   +4.31%  (p=0.028 n=17+20)
BM_eigen_v_Log2_f32/100   176ns ± 8%   180ns ± 7%   +2.19%  (p=0.045 n=16+17)
BM_eigen_v_Log2_f32/1k   1.44µs ± 4%  1.50µs ± 9%   +3.91%  (p=0.001 n=16+17)
BM_eigen_v_Log2_f32/10k  14.5µs ±10%  15.0µs ± 8%   +3.92%  (p=0.002 n=16+19)
```

Reviewed By: ezhulenev

Differential Revision: https://reviews.llvm.org/D98282