SVA: error message non-constant SVA cycle delays

Replicates third example in #931.

Author: kroening

regression/verilog/SVA/cycle_delay1.desc

@@ -0,0 +1,9 @@
+CORE
+cycle_delay1.sv
+
+^file cycle_delay1\.sv line \d+: expected constant expression, but got `main.from'$
+^EXIT=2$
+^SIGNAL=0$
+--
+^warning: ignoring
+--

regression/verilog/SVA/cycle_delay1.sv

@@ -0,0 +1,11 @@
+module main(input clk, input [31:0] from);
+
+  reg [31:0] x;
+
+  always_ff @(posedge clk)
+    x++;
+
+  // The cycle delay must be elaboration-time constant
+  initial assert property (##[from:2] x!=10);
+
+endmodule

src/verilog/verilog_typecheck_sva.cpp

@@ -193,11 +193,19 @@ exprt verilog_typecheck_exprt::convert_ternary_sva(ternary_exprt expr)
   if(expr.id() == ID_sva_cycle_delay) // ##[1:2] something
   {
     expr.type() = bool_typet();
+
     convert_expr(expr.op0());
+    elaborate_constant_expression_check(expr.op0());
+
     if(expr.op1().is_not_nil())
+    {
       convert_expr(expr.op1());
+      elaborate_constant_expression_check(expr.op1());
+    }
+
     convert_sva(expr.op2());
     make_boolean(expr.op2());
+
     return std::move(expr);
   }
   else if(