[SCEV] Recognize simple br-phi patterns

Summary:
Teach SCEV to match patterns like

```
  br %cond, label %left, label %right
 left:
  br label %merge
 right:
  br label %merge
 merge:
  V = phi [ %x, %left ], [ %y, %right ]
```

as "select %cond, %x, %y".  Before this SCEV would match PHI nodes
exclusively to add recurrences.

This addresses PR25005.

Reviewers: joker.eph, joker-eph, atrick

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D13378

llvm-svn: 249211