Add new utilities for RTTI Operation casting: dyn_cast_or_null and isa_nonnull

    * dyn_cast_or_null
      - This will first check if the operation is null before trying to 'dyn_cast':

        Value *v = ...;
        if (auto forOp = dyn_cast_or_null<AffineForOp>(v->getDefiningOp()))
          ...
    * isa_nonnull
      - This will first check if the pointer is null before trying to 'isa':

        Value *v = ...;
        if (isa_nonnull<AffineForOp>(v->getDefiningOp());
          ...

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PiperOrigin-RevId: 242171343