[ScopInfo] Completely remove MemoryAccesses when their parent statement is removed.

Removing a statement left its MemoryAccesses in some lists and maps of
the SCoP.  Which lists depends on at which phase of the SCoP
construction the statement is deleted.  Follow-up passes could still see
the already deleted MemoryAccesses by iterating through these
lists/maps, resulting in an access violation.

When removing a ScopStmt, also remove all its MemoryAccesses by using
the same mechnism that removes a MemoryAccess.

llvm-svn: 329640