[mlir] Add canonicalization for cond_br that feed into a cond_br on the same condition

```
   ...
   cond_br %cond, ^bb1(...), ^bb2(...)
 ...
 ^bb1: // has single predecessor
   ...
   cond_br %cond, ^bb3(...), ^bb4(...)
```

 ->

```
   ...
   cond_br %cond, ^bb1(...), ^bb2(...)
 ...
 ^bb1: // has single predecessor
   ...
   br ^bb3(...)
```

Differential Revision: https://reviews.llvm.org/D89604