[EarlyCSE] Make PhiToCheck in removeMSSA() a set.

This way we end up not looking at PHI args already removed.
MemSSA now goes through the updater so we can prune
it to avoid having redundant MemoryPHI arguments, but that
doesn't quite work for the general case.

Discussed with Daniel Berlin, fixes PR33406.

llvm-svn: 305409