Deduplicate constant folding logic in ConstantFold and GreedyPatternRewriteDriver

    There are two places containing constant folding logic right now: the ConstantFold
    pass and the GreedyPatternRewriteDriver. The logic was not shared and started to
    drift apart. We were testing constant folding logic using the ConstantFold pass,
    but lagged behind the GreedyPatternRewriteDriver, where we really want the constant
    folding to happen.

    This CL pulled the logic into utility functions and classes for sharing between
    these two places. A new ConstantFoldHelper class is created to help constant fold
    and de-duplication.

    Also, renamed the ConstantFold pass to TestConstantFold to make it clear that it is
    intended for testing purpose.

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PiperOrigin-RevId: 241971681