[prim_clock_mux] Model generic mux with boolean ops
[prim_clock_mux] Model generic mux with boolean ops

Since this primitive receives a don't touch constraint in GTECH
synthesis, we need to use the lowest level of abstraction here so
that the mapped GTECH netlist does not contain any higher level
constructs.

Signed-off-by: Michael Schaffner <a005a2d3342a687ea2d05360c953107acbe83e08@google.com>

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