opt_expr: simplify any unsigned comparisons with all-0 and all-1.

Before this commit, only unsigned comparisons with all-0 would be
simplified. This commit also makes the code handling such comparisons
to be more rigorous and not abort on unexpected input.

tests/opt/opt_expr_cmp.ys

@@ -0,0 +1,4 @@
+read_verilog opt_expr_cmp.v
+equiv_opt -assert opt_expr -fine
+design -load postopt
+select -assert-count 0 t:$gt t:$ge t:$lt t:$le