RegScavenging: Add scavengeRegisterBackwards()

Re-apply r276044/r279124. Trying to reproduce or disprove the ppc64
problems reported in the stage2 build last time, which I cannot
reproduce right now.

This is a variant of scavengeRegister() that works for
enterBasicBlockEnd()/backward(). The benefit of the backward mode is
that it is not affected by incomplete kill flags.

This patch also changes
PrologEpilogInserter::doScavengeFrameVirtualRegs() to use the register
scavenger in backwards mode.

Differential Revision: http://reviews.llvm.org/D21885

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@305516 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/CodeGen/RegisterScavenging.h

@@ -156,12 +156,24 @@ class RegScavenger {
   /// available and do the appropriate bookkeeping. SPAdj is the stack
   /// adjustment due to call frame, it's passed along to eliminateFrameIndex().
   /// Returns the scavenged register.
+  /// This is deprecated as it depends on the quality of the kill flags being
+  /// present; Use scavengeRegisterBackwards() instead!
   unsigned scavengeRegister(const TargetRegisterClass *RegClass,
                             MachineBasicBlock::iterator I, int SPAdj);
   unsigned scavengeRegister(const TargetRegisterClass *RegClass, int SPAdj) {
     return scavengeRegister(RegClass, MBBI, SPAdj);
   }
 
+  /// Make a register of the specific register class available from the current
+  /// position backwards to the place before \p To. If \p RestoreAfter is true
+  /// this includes the instruction following the current position.
+  /// SPAdj is the stack adjustment due to call frame, it's passed along to
+  /// eliminateFrameIndex().
+  /// Returns the scavenged register.
+  unsigned scavengeRegisterBackwards(const TargetRegisterClass &RC,
+                                     MachineBasicBlock::iterator To,
+                                     bool RestoreAfter, int SPAdj);
+
   /// Tell the scavenger a register is used.
   void setRegUsed(unsigned Reg, LaneBitmask LaneMask = LaneBitmask::getAll());
 
@@ -202,6 +214,12 @@ class RegScavenger {
 
   /// Mark live-in registers of basic block as used.
   void setLiveInsUsed(const MachineBasicBlock &MBB);
+
+  /// Spill a register after position \p After and reload it before position
+  /// \p UseMI.
+  ScavengedInfo &spill(unsigned Reg, const TargetRegisterClass &RC, int SPAdj,
+                       MachineBasicBlock::iterator After,
+                       MachineBasicBlock::iterator &UseMI);
 };
 
 /// Replaces all frame index virtual registers with physical registers. Uses the