relax restrictions on non-meta transformations

_From [bruen...@google.com](https://code.google.com/u/109494838902877177630/) on August 17, 2011 12:02:01_

Some restrictions cause clients to use meta instrs where really non-meta
should be used:
- Only one non-meta cbr per bb. My repstr loop transformation from
Dr. Memory and drutil is more complicated because of this.

Should also think about whether relaxing this restriction can result in
maintaining linear code more easily: repstr loop transformation is no
longer linear.

_Original issue: http://code.google.com/p/dynamorio/issues/detail?id=538_