[ManagedMemoryRewrite] Rewrite malloc, free correctly inside `Constant`s.

Reuse the machinery built for replacing global arrays to replace malloc/free as
well. Example replacement that was missed earlier:

```
call void \
    bitcast (void (i8*)* @free to void (%custom_type*)*) (%custom_type* %13)
```

- Since the `bitcast` is a `ConstantExpr`, `replaceAllUsesWith` would miss
this. We don't miss this anymore.

Differential Revision: https://reviews.llvm.org/D36825

llvm-svn: 311121