Unify the 'constantFold' and 'fold' hooks on an operation into just...

    Unify the 'constantFold' and 'fold' hooks on an operation into just 'fold'. This new unified fold hook will take constant attributes as operands, and may return an existing 'Value *' or a constant 'Attribute' when folding. This removes the awkward situation where a simple canonicalization like "sub(x,x)->0" had to be written as a canonicalization pattern as opposed to a fold.

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PiperOrigin-RevId: 248582024