[InstCombine] Set dead phi inputs to poison in more cases

Set phi inputs to poison whenever we find a dead edge (either
during initial worklist population or the main InstCombine run),
instead of only doing this for successors of dead blocks.

This means that the phi operand is set to poison even if for
critical edges without an intermediate block.

There are quite a few test changes, because the pattern is fairly
common in vectorizer output, for cases where we know the vectorized
loop will be entered.