[Reassociate] Avoid use of ConstantExpr::getShl()

Use the constant folding API instead.

Author: nikic

llvm/lib/Transforms/Scalar/Reassociate.cpp

@@ -1022,7 +1022,8 @@ static BinaryOperator *BreakUpSubtract(Instruction *Sub,
 static BinaryOperator *ConvertShiftToMul(Instruction *Shl) {
   Constant *MulCst = ConstantInt::get(Shl->getType(), 1);
   auto *SA = cast<ConstantInt>(Shl->getOperand(1));
-  MulCst = ConstantExpr::getShl(MulCst, SA);
+  MulCst = ConstantFoldBinaryInstruction(Instruction::Shl, MulCst, SA);
+  assert(MulCst && "Constant folding of immediate constants failed");
 
   BinaryOperator *Mul = BinaryOperator::CreateMul(Shl->getOperand(0), MulCst,
                                                   "", Shl->getIterator());