Assignments with an undefined intra-assignment delay should be treated like
assignments with zero delay. For the most part this is implemented
correctly, except for assignments to a part of a vector where the offset
inside the vector is an immediate value. E.g.
```
reg [1:0] x;
integer d = 'x;
...
x[0] <= #d 1'b1
```

Here when loading the delay into the index register flag 4 is updated, but
never cleared afterwards. As a result, if the delay is undefined, the vector
assignment will be skipped. Fix this by making sure flag 4 is always
cleared before the vector assignment instruction.

Signed-off-by: Lars-Peter Clausen <[email protected]>

Check that assignments with undefined intra-assignment delay get executed
with zero delay. The assignment should not be skipped.

Signed-off-by: Lars-Peter Clausen <[email protected]>