[TFLite] Fuse AddOp into preceding convolution ops

If we see an add op adding a constant value to a convolution op with constant
bias, we can fuse the add into the convolution op by constant folding the
bias and the add op's constant operand.

This CL also removes dangling RewriterGen check that prevents us from using
nested DAG nodes in result patterns, which is already supported.

PiperOrigin-RevId: 233989654