Add force_downto and force_upto wire attributes.

Fixes #2058.

tests/arch/xilinx/mux.ys

@@ -40,8 +40,10 @@ proc
 equiv_opt -assert -map +/xilinx/cells_sim.v synth_xilinx -noiopad # equivalency check
 design -load postopt # load the post-opt design (otherwise equiv_opt loads the pre-opt design)
 cd mux16 # Constrain all select calls below inside the top module
-select -assert-min 5 t:LUT6
+select -assert-max 2 t:LUT4
+select -assert-min 4 t:LUT6
 select -assert-max 7 t:LUT6
 select -assert-max 2 t:MUXF7
+dump
 
-select -assert-none t:LUT6 t:MUXF7 %% t:* %D
+select -assert-none t:LUT6 t:LUT4 t:MUXF7 %% t:* %D