ExecutionEngine: drop PassManager integration

    Originally, ExecutionEngine was created before MLIR had a proper pass
    management infrastructure or an LLVM IR dialect (using the LLVM target
    directly).  It has been running a bunch of lowering passes to convert the input
    IR from Standard+Affine dialects to LLVM IR and, later, to the LLVM IR dialect.
    This is no longer necessary and is even undesirable for compilation flows that
    perform their own conversion to the LLVM IR dialect.  Drop this integration and
    make ExecutionEngine accept only the LLVM IR dialect.  Users of the
    ExecutionEngine can call the relevant passes themselves.

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PiperOrigin-RevId: 249004676