fix(compiler): Batching: Bail out if batchable operand is not produced by an op

An early test for a batchable operation checks whether the batchable
operand is produced by a `tensor.extract` operation and bails out if
this is not the case. However, the use of `llvm::dyn_cast<T>()` directly
on the defining operation of the batchable operand causes an attempt
to cast a null value for an operand which is not produced by an
operation (e.g., block arguments).

Using `llvm::dyn_cast_or_null<T>()` fixes this issue.

compilers/concrete-compiler/compiler/lib/Transforms/Batching.cpp

@@ -578,7 +578,7 @@ public:
     // Find a batchable op which is embedded into a loop nest
     func.walk([&](BatchableOpInterface scalarOp) {
       // Is producer an extract op?
-      auto extractOp = llvm::dyn_cast<mlir::tensor::ExtractOp>(
+      auto extractOp = llvm::dyn_cast_or_null<mlir::tensor::ExtractOp>(
           scalarOp.getBatchableOperand().get().getDefiningOp());
 
       if (!extractOp)