[mlir] Clarify operation/block assert message.

[matthiaskramm]

Summary

When you try to add the same operation into a block twice, we used
to print "already in an operation block", which sounded misleading.
(Sound like e.g. our builder insertion point is inside a block.)
Instead, make it clear that the operation is being used twice.

mlir/lib/IR/Operation.cpp

Block *llvm::ilist_traits<::mlir::Operation>::getContainingBlock() {
   size_t Offset(size_t(&((Block *)nullptr->*Block::getSublistAccess(nullptr))));
   iplist<Operation> *Anchor(static_cast<iplist<Operation> *>(this));
   return reinterpret_cast<Block *>(reinterpret_cast<char *>(Anchor) - Offset);
 }
 
 /// This is a trait method invoked when an operation is added to a block.  We
 /// keep the block pointer up to date.
 void llvm::ilist_traits<::mlir::Operation>::addNodeToList(Operation *op) {
-  assert(!op->getBlock() && "already in an operation block!");
+  assert(!op->getBlock() && "operation already exists in some block!");
   op->block = getContainingBlock();
 
   // Invalidate the order on the operation.
   op->orderIndex = Operation::kInvalidOrderIdx;
 }
 
 /// This is a trait method invoked when an operation is removed from a block.
 /// We keep the block pointer up to date.