xilinx: Fix srl regression.

xilinx: Fix srl regression.

Of standard yosys cells, xilinx_srl only works on $_DFF_?_ and
$_DFFE_?P_, which get upgraded to $_SDFFE_?P?P_ by dfflegalize at the
point where xilinx_srl is called for non-abc9.  Fix this by running
ff_map.v first, resulting in FDRE cells, which are handled correctly.