tgt-vvp: Fix load skip for assignment operator to array entry

The vvp `%load/vec4a` instruction will skip the load if vvp flag 4 is set
and return 'x. This is meant for handling the case where the index is
undefined.

For assignment operators on array entries, when the index is an immediate
value, vvp flag 4 is not cleared before the load instruction. If a previous
instruction set flag 4 it load yield 'x.

E.g. for the following sequence `x[0]` should be `11`, but will be `'x`.

```
integer x[10];
logic a = 1'b0;
x[0] = 10;
if (a == 0) begin
  x[0] += 1;
end
```

Properly clear the flag before the load instruction to handle this
correctly.

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

tgt-vvp/stmt_assign.c

@@ -161,6 +161,7 @@ static void get_vec_from_lval_slice(ivl_lval_t lval, struct vec_slice_info*slice
 	    if (use_word < ivl_signal_array_count(sig)) {
 		  fprintf(vvp_out, "    %%ix/load 3, %lu, 0;\n",
 			  use_word);
+		  fprintf(vvp_out, "    %%flag_set/imm 4, 0;\n");
 		  fprintf(vvp_out, "    %%load/vec4a v%p, 3;\n", sig);
 	    } else {
 		  assert(wid <= 32);