Change eliminateCallFramePseudoInstr() to return an iterator

This will become necessary in a subsequent change to make this method
merge adjacent stack adjustments, i.e. it might erase the previous
and/or next instruction.

It also greatly simplifies the calls to this function from Prolog-
EpilogInserter. Previously, that had a bunch of logic to resume iteration
after the call; now it just continues with the returned iterator.

Note that this changes the behaviour of PEI a little. Previously,
it attempted to re-visit the new instruction created by
eliminateCallFramePseudoInstr(). That code was added in r36625,
but I can't see any reason for it: the new instructions will obviously
not be pseudo instructions, they will not have FrameIndex operands,
and we have already accounted for the stack adjustment.

Differential Revision: http://reviews.llvm.org/D18627

llvm-svn: 265036
diff --git a/llvm/lib/Target/BPF/BPFFrameLowering.h b/llvm/lib/Target/BPF/BPFFrameLowering.h
index 251cda9..5db963f 100644
--- a/llvm/lib/Target/BPF/BPFFrameLowering.h
+++ b/llvm/lib/Target/BPF/BPFFrameLowering.h
@@ -31,10 +31,10 @@
   void determineCalleeSaves(MachineFunction &MF, BitVector &SavedRegs,
                             RegScavenger *RS) const override;
 
-  void
+  MachineBasicBlock::iterator
   eliminateCallFramePseudoInstr(MachineFunction &MF, MachineBasicBlock &MBB,
                                 MachineBasicBlock::iterator MI) const override {
-    MBB.erase(MI);
+    return MBB.erase(MI);
   }
 };
 }