[TargetFrameLowering] Refactor `determineCalleeSaves`.
Split up `determineCaleeSaves` into:
`const MCPhysReg *getMustPreserveRegisters(MachineFunction &MF) const`:
Return the list of registers which must be preserved by the function: the value on exit must be the same as the value on entry. A register from this list does not need to be saved / reloaded if the function did not use it.
`virtual void determineUncondPrologCalleeSaves(MachineFunction &MF, const MCPhysReg *CSRegs, BitVector &UncondPrologCSRs) const`:
Determines which of the registers reported by getMustPreserveRegisters() must be saved in prolog and reloaded in epilog regardless of wheather or not they were modified by the function.
`virtual void determineEarlyCalleeSaves(MachineFunction &MF, BitVector &EarlyCSRs)`:
Returns the difference between getMustPreserveRegisters and determineUncondPrologCalleeSaves. These registers will be preserved by the code optimizer and do not need to be saved in prolog.
`virtual void determinePrologCalleeSaves(MachineFunction &MF,
BitVector &PrologCSRs,
RegScavenger *RS) const`:
This method returns those registers in the difference of getMustPreserveRegisters and determineEarlyCalleeSaves that were modified by the function and need to be saved in prolog.
