LoopsToGPU: use PassRegistration with constructor

PassRegistration with an optional constructor was introduced after the
LoopsToGPUPass, which resorted to deriving one pass from another as a means of
accepting options supplied as command-line arguments. Use PassRegistration with
constructor instead of defining a derived pass for LoopsToGPU.  Also rename the
pass to better reflect its current nature.

PiperOrigin-RevId: 257786923