Cmm: track the type of global registers

This is an attempt to keep track of the types we store in Cmm global registers. This is meant to address #22297 (closed). Putting it up for preliminary discussion only, for the moment.

TODO:

• Refactor the VanillaReg constructor of GlobalReg by removing the VGcPtr field. This information should instead be recovered from the CmmType of the GlobalRegUse.
• Make AvailRegs into a datatype.