tgt-vvp: Handle dynamic part select on array element non-blocking event control

VVP array assignment operations expect the array element index to be in
index register 3.

For array element assignments with a dynamic part select the array index
gets moved into a temporary index register and has to be moved into
register 3 after evaluating the dynamic part select.  This is currently not
done non-blocking event control assignments. This causes the write to go to
the wrong array element. It will go to whatever value is in the register 3
from previous operations.

```
reg [3:0] a[1:0];
integer i = 0;
event e;
a[1][i+:2] <= @e 2'b10; // Will write to the wrong array element
->e;
```

Make sure to move the temporary register to register 3.

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

tgt-vvp/vvp_process.c

@@ -179,6 +179,8 @@ static void assign_to_array_word(ivl_signal_t lsig, ivl_expr_t word_ix,
 	      /* If needed use the global error state. */
 	    if (part_off_ex) {
 		  fprintf(vvp_out, "    %%flag_mov 4, %d;\n", error_flag);
+		  fprintf(vvp_out, "    %%ix/mov 3, %d;\n", word_ix_reg);
+		  clr_word(word_ix_reg);
 	    }
 	    fprintf(vvp_out, "    %%assign/vec4/a/e v%p, %d;\n", lsig, part_off_reg);