[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

include/llvm/ADT/STLExtras.h

@@ -1123,10 +1123,13 @@ public:
   }
 
   /// Compare this range with another.
-  template <typename OtherT> bool operator==(const OtherT &other) {
+  template <typename OtherT> bool operator==(const OtherT &other) const {
     return size() == std::distance(other.begin(), other.end()) &&
            std::equal(begin(), end(), other.begin());
   }
+  template <typename OtherT> bool operator!=(const OtherT &other) const {
+    return !(*this == other);
+  }
 
   /// Return the size of this range.
   size_t size() const { return count; }