PrologEpilogInserter: Rewrite API to determine callee save regsiters.

PrologEpilogInserter: Rewrite API to determine callee save regsiters.

This changes TargetFrameLowering::processFunctionBeforeCalleeSavedScan():

- Rename the function to determineCalleeSaves()
- Pass a bitset of callee saved registers by reference, thus avoiding
  the function-global PhysRegUsed bitset in MachineRegisterInfo.
- Without PhysRegUsed the implementation is fine tuned to not save
  physcial registers which are only read but never modified.

Related to rdar://21539507

Differential Revision: http://reviews.llvm.org/D10909

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@242165 91177308-0d34-0410-b5e6-96231b3b80d8