[mlir][Standard] Add a canonicalization to simplify cond_br when the successors are identical

This revision adds support for canonicalizing the following:

```
cond_br %cond, ^bb1(A, ..., N), ^bb1(A, ..., N)

br ^bb1(A, ..., N)
```

 If the operands to the successor are different and the cond_br is the only predecessor, we emit selects for the branch operands.

```
cond_br %cond, ^bb1(A), ^bb1(B)

%select = select %cond, A, B
br ^bb1(%select)
```

Differential Revision: https://reviews.llvm.org/D78682