[MLIR][IR] Add assert to Operation::moveBefore (NFC) (llvm#137772)

This commit adds an assert to `Operation::moveBefore` which ensures that
moving of operations without a parent block is detected early on. This
case otherwise runs into a segfault, as it's assumed that there is an
parent block.

Author: Dinistro

mlir/lib/IR/Operation.cpp

@@ -322,8 +322,8 @@ void Operation::setAttrs(DictionaryAttr newAttrs) {
 }
 void Operation::setAttrs(ArrayRef<NamedAttribute> newAttrs) {
   if (getPropertiesStorageSize()) {
-    // We're spliting the providing array of attributes by removing the inherentAttr
-    // which will be stored in the properties.
+    // We're spliting the providing array of attributes by removing the
+    // inherentAttr which will be stored in the properties.
     SmallVector<NamedAttribute> discardableAttrs;
     discardableAttrs.reserve(newAttrs.size());
     for (NamedAttribute attr : newAttrs) {
@@ -560,6 +560,8 @@ void Operation::moveBefore(Operation *existingOp) {
 /// before `iterator` in the specified basic block.
 void Operation::moveBefore(Block *block,
                            llvm::iplist<Operation>::iterator iterator) {
+  assert(getBlock() &&
+         "cannot move an operation that isn't contained in a block");
   block->getOperations().splice(iterator, getBlock()->getOperations(),
                                 getIterator());
 }