Update the implicit global detection by looking for them in the CUF
kernel and also update to a walk so nested `fir.address_of` in nested
statement are also accounted for.
For consistency with other dialects and other CUF passes and files, this
patch renames passes CufOpConversion to CUFOpConversion,
CufImplicitDeviceGlobal to CUFDeviceGlobal.
It also renames the file.
