Since a CmmCall returns CmmFormals which may include
global registers (and indeed one place in the code
returns the results of a CmmCall into BaseReg) and
since CPS conversion will change those return slots
into formal arguments for the continuation of the call,
CmmProc has to have CmmFormals for the formal arguments.

Oddly, the old code never made use of procedure arguments
so this change only effects the types and not any of the code.
(Because [] is both of type [LocalReg] and CmmFormals.)