Fix store to opaque ptr

[alexander-shaposhnikov]

If ptr.type.is_opaque is True it doesn't need to have ptr.type.pointee attribute.
Add a unit test.

[sklam]

@rj-jesus can you review this?

[rj-jesus]

@sklam Yep, sure.
Thanks for the patch, @alexander-shaposhnikov.
In general it LGTM, my only comment would be that the new test test_store could maybe be part of test_mem_ops, but I don't think this is critical.

[alexander-shaposhnikov]

Many thanks, I've updated the PR.

[rj-jesus]

Thanks very much, LGTM!
(@sklam I can't approve it myself.)

[alexander-shaposhnikov]

@sklam (would be amazing if you could merge this in, many thanks in advance)