[mlir][llvm] Drop unreachable basic block during import (#78467) This revision updates the LLVM IR import to support unreachable basic blocks. An unreachable block may dominate itself and a value defined inside the block may thus be used before its definition. The import does not support such dependencies. We thus delete the unreachable basic blocks before the import. This is possible since MLIR does not have basic block labels that can be reached using an indirect call and unreachable blocks can indeed be deleted safely. Additionally, add a small poison constant import test.
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