[InstCombine] Fix handling of irreducible loops (PR64259)

Fixes a regression introduced by D75362 for irreducible control
flow. In that case, we may visit the predecessor that renders
the current block live only later, and incorrectly determine
that a block is dead.

Instead, switch to using the same DeadEdges based implementation
we also use during the main InstCombine iteration.

This temporarily regresses some cases that need replacement of
dead phi operands with poison, which is currently only done during
the main run, but not worklist population. This will be addressed
in a followup, to keep it separate from the correctness fix here.

Fixes https://github.com/llvm/llvm-project/issues/64259.