[mlir][arith] Add canonicalization patterns for 'mul*i_extended'

- Add a fold for `mulsi_extended(x, 1)`
- Add folds to demote wide integer multiplication to `mul*i_extended` when the result is shifted
   and truncated: `trunci(shrui(mul(*ext(x), *ext(y)), c)) -> mul*i_extended(x, y)`

Reviewed By: Mogball

Differential Revision: https://reviews.llvm.org/D139778