[mlir][arith] Improve `extf` folder (#80232)

* Use APFloat conversion function to avoid losing information by
converting to `double`. This would be the case with large types like
`f80` or `f128`.
* Check for potential information loss. This is intended for small
floating point types that may have values not present in larger ones
(e.g., f8m2e5fnuz and f16).
* Support folding vector constants.