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
af331bc52d