[PATCH] Implement variable-sized alloca instrumentation.

[Yury Gribov]

Actually exit domination wouldn't save the day. E.g.
  void f() {
    char *p;
    ...
    do {
      p = alloca(100);
      g(p);
    } while(whatever);
    ...
  }

http://reviews.llvm.org/D6055