Function dt_cg_ternary_op

* Code generation for the ternary op requires some trickery with the assembler * in order to conserve registers. We generate code for dn_expr and dn_left * and free their registers so they do not have be consumed across codegen for * dn_right. We insert a dummy MOV at the end of dn_left into the destination * register, which is not yet known because we haven't done dn_right yet, and * save