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

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@208492 91177308-0d34-0410-b5e6-96231b3b80d8

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