LivePhysRegs: Fix addLiveOutsNoPristines() for return blocks past PEI

Re-commit r303938 and r303954 with a fix for addLiveIns(): the internal
addPristines() function must be called on an empty set or it may
accidentally reset saved registers.

- addLiveOutsNoPristines() needs to add callee saved registers that are
  actually saved and restored somewhere to the set (they are not
  pristine).
- Cleanup/rewrite the code for addLiveOuts()/addLiveOutsNoPristines().

This fixes the problem from D32156.

Differential Revision: https://reviews.llvm.org/D32464

llvm-svn: 304001