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This version should compile but is still incomplete as it introduces
potential bugs at the places marked 'TODO FIXME NOW'.
It is being recorded to help keep track of changes.

The return values were getting put in a LocalReg
but that LocalReg needed to be stored into the enclosing
expression's return register (e.g. R1).

This patch is incomplete and will probably not compile by itself
but it is being recorded to help keep track of later changes.

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.

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.)

The function
  listSplitUniqSupply :: UniqSupply -> [UniqSupply]
splits a UniqSupply into an infinite list of supplies.
It is a generalization of splitUniqSupply.

This is a safety check-in, before I begin a cleanup.