Fixes following the CR by Chris and Duncan:

Optimize chained bitcasts of the form A->B->A.
Undo r138722 and change isEliminableCastPair to allow this case.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@138756 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Analysis/ConstantFolding.cpp

@@ -51,12 +51,6 @@ static Constant *FoldBitCast(Constant *C, Type *DestTy,
   if (C->isAllOnesValue() && !DestTy->isX86_MMXTy())
     return Constant::getAllOnesValue(DestTy);
 
-  // Bitcast of Bitcast can be done using a single cast.
-  ConstantExpr *CE = dyn_cast<ConstantExpr>(C);
-  if (CE && CE->getOpcode() == Instruction::BitCast) {
-    return ConstantExpr::getBitCast(CE->getOperand(0), DestTy);
-  }
-
   // The code below only handles casts to vectors currently.
   VectorType *DestVTy = dyn_cast<VectorType>(DestTy);
   if (DestVTy == 0)