Add support for null coalesce assign operator

[thegallagher]

No description provided.

[nikic]

This is not correct, because it doesn't model the short-circuiting behavior. But the existing ?? handling doesn't do so either, so I guess this is fine as a starting point.

[thegallagher]

Thanks for the feedback. I assume the correct behaviour would be to expand null coalesce to JumpIf, as with LogicalAnd and LogicalOr?