MachineFrameInfo: Simplify pristine register calculation.

About pristine regsiters:
Pristine registers "hold a value that is useless to the current
function, but that must be preserved - they are callee saved registers
that have not been saved." This concept saves compile time as it frees
the prologue/epilogue inserter from adding every such register to every
basic blocks live-in list.

However the current code in getPristineRegs is formulated in a
complicated way: Inside the function prologue and epilogue all callee
saves are considered pristine, while in the rest of the code only the
non-saved ones are considered pristine.  This requires logic to
differentiate between prologue/epilogue and the rest and in the presence
of shrink-wrapping this even becomes complicated/expensive.  It's also
unnecessary because the prologue epilogue inserters already mark
callee-save registers that are saved/restores properly in the respective
blocks in the prologue/epilogue (see updateLiveness() in
PrologueEpilogueInserter.cpp). So only declaring non-saved/restored
callee saved registers as pristine just works.

Differential Revision: http://reviews.llvm.org/D10101

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@238524 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/MachineVerifier.cpp

@@ -694,7 +694,7 @@ MachineVerifier::visitMachineBasicBlockBefore(const MachineBasicBlock *MBB) {
 
   const MachineFrameInfo *MFI = MF->getFrameInfo();
   assert(MFI && "Function has no frame info");
-  BitVector PR = MFI->getPristineRegs(MBB);
+  BitVector PR = MFI->getPristineRegs(*MF);
   for (int I = PR.find_first(); I>0; I = PR.find_next(I)) {
     for (MCSubRegIterator SubRegs(I, TRI, /*IncludeSelf=*/true);
          SubRegs.isValid(); ++SubRegs)