Move MRI liveouts to XCore return instructions.
Move MRI liveouts to AArch64 return instructions.
Remove special-casing of return blocks for liveness.

Now that return value registers are return instruction uses, there is no
need for special treatment of return blocks.
Remove liveout lists from MachineRegisterInfo.

All targets are now adding return value registers as implicit uses on
return instructions, and there is no longer a need for the live out
lists.