Reapply [InstCombine] Support switch in phi to cond fold

Reapply with an explicit check for multi-edges, as the expected
behavior of multi-edge dominance is unclear (D120811).

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For conditional branches, we know the value is i1 0 or i1 1 along
the outgoing edges. For switches we can apply exactly the same
optimization, just with the known values determined by the switch
cases.