[MLIR] Fix dialect conversion cancelRootUpdate

Fix dialect conversion ConversionPatternRewriter::cancelRootUpdate: the
erasure of operations here from the list of root update was off by one.
Should have been:
```
rootUpdates.erase(rootUpdates.begin() + (rootUpdates.rend() - it - 1));
```
instead of
```
rootUpdates.erase(rootUpdates.begin() + (rootUpdates.rend() - it));
```

or more directly:
```
rootUpdates.erase(it.base() - 1)
```

While on this, add an assertion to improve dev experience when a cancel is
called on an op on which a root update hasn't been started.

Differential Revision: https://reviews.llvm.org/D105397