[DSE,MemorySSA] Return early when hitting a MemoryPhi.

A MemoryPhi can never be eliminated. If we hit one, return the Phi, so
the caller can continue traversing the incoming accesses.

This saves some unnecessary read clobber checks and improves
compile-time
http://llvm-compile-time-tracker.com/compare.php?from=1ffc58b6d098ce8fa71f3a80fe75b990f633f921&to=d0fa8d1982380b57d7b6067528104bc373dbe07a&stat=instructions