hexagon: various Op fixes (llama/17135)

hexagon: various Op fixes (llama/17135)

* hexagon: explicitly check for ops with zero nrows

llm_graph_context::build_inp_out_ids() can generate tensors with zero nrows.
Somehow other backends seems to handle this without obvious explicit checks.
In the hexagon case we need to check explicitly and skip them.

* hexagon: introduce fastdiv, fix test-backend-ops for ADD/SUB/MUL

Co-authored-by: chraac <redacted>
* hexagon: use fastdiv in ADD_ID

* hexagon: use ggml_op_is_empty and ggml_is_empty to check for NOPs

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Co-authored-by: chraac <redacted>