Update to match llvm r280032

Use the ilist getReverse() function instead of constructing a
reverse_iterator from a forward iterator.

Author: bob-wilson

lib/SILOptimizer/Analysis/ARCAnalysis.cpp

@@ -1136,8 +1136,7 @@ SILInstruction *swift::findReleaseToMatchUnsafeGuaranteedValue(
       RCFI.getRCIdentityRoot(UnsafeGuaranteedI->getOperand(0));
 
   // Look before the "unsafeGuaranteedEnd".
-  for (auto ReverseIt = SILBasicBlock::reverse_iterator(
-                UnsafeGuaranteedEndI->getIterator()),
+  for (auto ReverseIt = ++UnsafeGuaranteedEndI->getIterator().getReverse(),
             End = BB.rend();
        ReverseIt != End; ++ReverseIt) {
     SILInstruction &CurInst = *ReverseIt;

lib/SILOptimizer/LoopTransforms/COWArrayOpt.cpp

@@ -907,7 +907,7 @@ bool COWArrayOpt::isArrayValueReleasedBeforeMutate(
 }
 
 static SILInstruction *getInstBefore(SILInstruction *I) {
-  auto It = SILBasicBlock::reverse_iterator(I->getIterator());
+  auto It = ++I->getIterator().getReverse();
   if (I->getParent()->rend() == It)
     return nullptr;
   return &*It;
@@ -939,8 +939,7 @@ stripValueProjections(SILValue V,
 /// by the array bounds check elimination pass.
 static SILInstruction *
 findPrecedingCheckSubscriptOrMakeMutable(ApplyInst *GetElementAddr) {
-  for (auto ReverseIt =
-           SILBasicBlock::reverse_iterator(GetElementAddr->getIterator()),
+  for (auto ReverseIt = ++GetElementAddr->getIterator().getReverse(),
             End = GetElementAddr->getParent()->rend();
        ReverseIt != End; ++ReverseIt) {
     auto Apply = dyn_cast<ApplyInst>(&*ReverseIt);

lib/SILOptimizer/SILCombiner/SILCombinerMiscVisitors.cpp

@@ -857,8 +857,7 @@ SILCombiner::visitInjectEnumAddrInst(InjectEnumAddrInst *IEAI) {
         return nullptr;
       }
 
-      for (llvm::iplist<SILInstruction>::reverse_iterator InsIt(
-               CurrIns->getIterator());
+      for (auto InsIt = ++CurrIns->getIterator().getReverse();
            InsIt != CurrBB->rend(); ++InsIt) {
         SILInstruction *Ins = &*InsIt;
         if (Ins == DataAddrInst) {