[mlir][Standard] Add canonicalization for collapsing pass through cond_br successors.

This revision adds support for the following canonicalization:

```
   cond_br %cond, ^bb1, ^bb2
 ^bb1
   br ^bbN(...)
 ^bb2
   br ^bbK(...)

   cond_br %cond, ^bbN(...), ^bbK(...)
```

Differential Revision: https://reviews.llvm.org/D78681