[MLIR][Arith] add fastMathAttr on arith::extf and arith::truncf (#93443)

Add an `fastMathAttr` on `arith::extf` and `arith::truncf`. If these two
ops are inserted by some promotion passes (like legalize-to-f32 /
emulate-unsupported-floats), they will be labeled as
`FastMathFlags::contract`, denoting that they can be then `eliminated by
canonicalizer`.

The `elimination` can help improve performance, while may introduce some
numerical differences.