[RS4GC] Remove an overly strong assertion

As shown by the included test case, it's reasonable to end up with constant references during base pointer calculation.  The code actually handled this case just fine, we only had the assert to help isolate problems under the belief that constant references shouldn't be present in IR generated by managed frontends. This turned out to be wrong on two fronts: 1) Manual Jacobs is working on a language with constant references, and b) we found a case where the optimizer does create them in practice.

llvm-svn: 256079