tgt-vvp: Handle signedness when passing struct member to system function

Access to members in packed struct fields is internally implemented using a
part select.

vvp has a special syntax for passing a part select of a vector to a system
function. This special syntax assumes that the part select is unsigned like
it is for normal Verilog part selects.

As a result passing a signed struct member to a system function will
interpret it as unsigned.

Add a check to make sure that the expression is actually unsigned. If it is
not fall back to evaluating the expression on the vector stack and pass the
value on the stack to the system function.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>

tgt-vvp/draw_vpi.c

@@ -199,6 +199,11 @@ static int get_vpi_taskfunc_signal_arg(struct args_info *result,
 	    if (ivl_expr_value(vexpr) == IVL_VT_DARRAY)
 		  return 0;
 
+	      /* Part select is always unsigned. If the expression is signed
+	       * fallback. */
+	    if (ivl_expr_signed(expr))
+		  return 0;
+
 	      /* The signal is part of an array. */
 	      /* Add &APV<> code here when it is finished. */
 	    bexpr = ivl_expr_oper2(expr);