[DSE,MemorySSA] Account for ScanLimit == 0 on entry.

Currently the code does not account for the fact that getDomMemoryDef
can be called with ScanLimit == 0, if we reached the limit while
processing an earlier access. Also tighten the check a bit more and bump
the scan limit now that it is handled properly.

In some cases, this brings a 2x speedup in terms of compile-time.