This commit align the implementation of
`ConversionPatternRewriter::legalize` with its documentation:
```
/// Attempt to legalize the given region. This can be used within
...
LogicalResult legalize(Region *r);
```
This function now legalizes the entire region, including nested ops. The
implementation follows the same logic as the "main" traversal:
pre-order, forward-dominance.
78711b66bd