[mlir][arith] Add narrowing patterns for `addi` and `muli`

These two ops are handled in a very similar way -- the only difference
in the number result bits produced.

I checked these transformation with Alive2:
1.  addi + sext: https://alive2.llvm.org/ce/z/3NSs9T
2.  addi + zext: https://alive2.llvm.org/ce/z/t7XHOT
3.  muli + sext: https://alive2.llvm.org/ce/z/-7sfW9
4.  muli + zext: https://alive2.llvm.org/ce/z/h4yntF

Reviewed By: antiagainst

Differential Revision: https://reviews.llvm.org/D149530