[FIX] Erase stall results during the SCoP detection

  With this patch we erase cached results for regions that are invalid
  as early as possible. If we do not (as before), it is possible that
  two expanded regions will have the same address and the tracked
  results for both are mixed. Currently this would "only" cause
  pessimism in later passes but that will change when we allow invariant
  loads in the SCoP.  Additionally, it triggers non-deterministic
  results as we might dismiss a later region because of results cached
  for an earlier one.

  There is no test case as the problem occurs only non-deterministically.

llvm-svn: 249000