Remove the explicit SUnitIterator::operator= as the default is just fine

Remove the explicit SUnitIterator::operator= as the default is just fine

There doesn't seem to be any need to assert that iterator assignment is
between iterators over the same node - if you want to reuse an iterator
variable to iterate another node, that's perfectly acceptable. Just
don't mix comparisons between iterators into disjoint sequences, as
usual.

llvm-svn: 231134