Accept additional conversions in the LLVM lowering

    Extend the LLVM lowering class following the original idea of the "bag of
    conversions".  LLVMLowering class is now exposed as and can be derived from.
    It provides hooks for derived classes to inject operation conversions and to
    convert custom types.  It is under responsibility of the caller to make sure
    patterns don't overlap.

    Update the lowering from the Linalg dialect to the LLVM IR dialect to use this
    new approach.

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PiperOrigin-RevId: 246492919