[mlir][arith] Add wide integer emulation pass

In this first patch in a series to add wide integer emulation:
*  Set up the initial pass structure
*  Add a custom type converter
*  Handle func ops

The initial implementation supports power-of-two integers types only. We
emulate wide integer operations by splitting original i2N integer types
into two iN halves

My immediate use case is to emulate i64 operations using i32 ones
on mobile GPUs that do not support i64.

Reviewed By: antiagainst, Mogball

Differential Revision: https://reviews.llvm.org/D133135