SLPVectorizer: Instead of just performing CSE on dead blocks ignore them completely.

Turns out that there is a very cheap way of testing whether a block is dead,
just look it up in the DomTree. We have to do this anyways so just ignore
unreachable blocks before sorting by domination. This restores a proper
ordering for std::stable_sort when dead code is present.

Covered by existing tests & buildbots running in STL debug mode (MSVC).