[DAGCombiner][RISCV] Preserve disjoint flag in folding (shl (or x, c1), c2) ->...

[DAGCombiner][RISCV] Preserve disjoint flag in folding (shl (or x, c1), c2) -> (or (shl x, c2), c1 << c2) (#76860)

Since we are shifting both inputs to the original Or by the same amount
and inserting zeros in the LSBs, the result should still be disjoint.