[Hexagon] Fix shift value when folding shl DAG node (#83853)

When folding (or (shl xx, s), (zext y)) to (COMBINE (shl xx, s-32), y),
fix resulting shift value in HexagonISD::COMBINE node to not generate
negative values.

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Co-authored-by: Yashas Andaluri <yandalur@qti.qualcomm.com>