[Bugfix] Prevent casting handle to other types

[syang-ng]

As we discussed in this post, a casting from handle to int or other types is actually undefined.

For example, when we try to cast a handle to bool, it will invoke llvm::Constant* zero = llvm::ConstantInt::get(DTypeToLLVMType(from), 0);, which means that we regard the from type is int, but actually it is handle.

src/target/llvm/codegen_llvm.cc - CodeGenLLVM::CreateCast:

llvm::Value* CodeGenLLVM::CreateCast(DataType from, DataType to, llvm::Value* value) {
  llvm::Type* target = DTypeToLLVMType(to);
  if (value->getType() == target) return value;
  if (to.is_handle()) {
    return builder_->CreateBitCast(value, target);
  } else if (to.is_uint() && to.bits() == 1) {
    if (from.is_float()) {
      llvm::Constant* zero = llvm::ConstantFP::get(DTypeToLLVMType(from), 0.);
      return builder_->CreateFCmpONE(value, zero);
    } else {
      llvm::Constant* zero = llvm::ConstantInt::get(DTypeToLLVMType(from), 0);
      return builder_->CreateICmpNE(value, zero);
    }
  } else if (!from.is_float() && !to.is_float()) {
    return builder_->CreateIntCast(value, target, from.is_int());
  } else if (from.is_float() && to.is_int()) {
    return builder_->CreateFPToSI(value, target);
  } else if (from.is_float() && to.is_uint()) {
    if (to.bits() < 8) {
      value = builder_->CreateFPToUI(value, DTypeToLLVMType(to.with_bits(8)));
      return builder_->CreateIntCast(value, target, false);
    } else {
      return builder_->CreateFPToUI(value, target);
    }
  } else if (from.is_int() && to.is_float()) {
    return builder_->CreateSIToFP(value, target);
  } else if (from.is_uint() && to.is_float()) {
    return builder_->CreateUIToFP(value, target);
  } else {
    ICHECK(from.is_float() && to.is_float());
    return builder_->CreateFPCast(value, target);
  }
}

And this undefined behavior may cause some other problems, like program crashes: