[MLIR][Linalg] Left over Linalg named ops from previous PR (#90405)

Adding `erf` as unary and `powf` as binary.

Same as `max(arg, 0.0)` for `ReLU`, `powf(arg, const)` can be either a
generic (with broadcast) or a pair (`linalg.broadcast + linalg.powf`)
and then lowered "correctly". Either way, the lower dialects need to
know what kind of broadcast anyway, so no materialization of the
constant tensors should remain.

I want to flush the easy ones before we start working on type cast &
softmax.