[TableGen] Allocate `Operator` object on heap in `RecordOperatorMap`

    Iterators for a `llvm::DenseMap` can be invalidated when an insertion occurs.
    In Pattern's `collectBoundArguments()`, we recursively handle all nested DAG
    nodes and grow the the `RecordOperatorMap`, while retaining a reference.
    This can cause the reference to be invalid and the program to behave randomly.
    Allocate each `Operator` object specifically to solve this issue.

    Also, `llvm::DenseMap` is a great way to map pointers to pointers, or map
    other small types to each other. This avoids placing the `Operator` object
    directly into the map.

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PiperOrigin-RevId: 243281486