This frees the Cmm data type from keeping a list of live global registers
in CmmCall which helps prepare for the CPS conversion phase.

CPS conversion does its own liveness analysis and takes input that should
not directly refer to parameter registers (e.g. R1, F5, D3, L2).  Since
these are the only things which could occur in the live global register
list, CPS conversion makes that field of the CmmCall constructor obsolite.

Once the CPS conversion pass is fully implemented, global register saving
will move from codeGen into the CPS pass.  Until then, this patch
is worth scrutinizing and testing to ensure it doesn't cause any performance
or correctness problems as the code passed to the backends by the CPS
converting will look very similar to the code that this patch makes codeGen
pass to the backend.