[mlir][arith] Add `exact` to `index_cast{,ui}` (#183395)
The `exact` flag with the following semantics
> If the `exact` attribute is present, it is assumed that the index type
width
> is such that the conversion does not lose information. When this
assumption
> is violated, the result is poison.
can be added to index_cast and index_castui operations. This unlocks
the following lowerings:
* index_cast (signed) exact -> trunc nsw
* index_castui (unsigned) exact -> trunc nuw
* index_castui nneg exact -> trunc nuw nsw
Changes:
* Adds ArithExactFlagInterface.
* Updates Arith_IntBinaryOpWithExactFlag to use ArithExactFlagInterface
* Update IndexCastOp and IndexCastUIOp to declare
`ArithExactFlagInterface`
* Update canonicalization patterns
* Update roundtrip, lowering, and canonicalization tests.
