[Standard] Add `atan2` to the standard dialect

[mehdi_amini]

@bondhugula

For eg. you can depend on MLIRAffineOps without depending on MLIRAffineTransforms

I don’t consider MLIRAffineTransforms to be “part of the Dialect”.
I’m referring to the Dialect as in the C++ class and all of its dependency: this is already too big.