[Constant Hoisting] Erase dead cast instructions.

The cleanup code that removes dead cast instructions only removed them from the
basic block, but didn't delete them. This fix erases them now too.

llvm-svn: 204538

llvm/lib/Transforms/Scalar/ConstantHoisting.cpp

@@ -595,7 +595,7 @@ bool ConstantHoisting::emitBaseConstants() {
 void ConstantHoisting::deleteDeadCastInst() const {
   for (auto const &I : ClonedCastMap)
     if (I.first->use_empty())
-      I.first->removeFromParent();
+      I.first->eraseFromParent();
 }
 
 /// \brief Optimize expensive integer constants in the given function.