[llvm] Image attribute access for the R600 backend

[Zoltán Gilián]

Thanks for the feedback! I've already added some tests in the
meantime, and I'll fix the other issues.

> Is this patch supposed to be functional as is, and on what hardware?

I'm testing on a juniper chip (driver support is a work in progress),
so I'm currently targeting evergreen hardware.

> Also this won’t correctly handle image handles passed to other functions.

Every function gets inlined, since the hardware doesn't support
function calls. As far as I know the pass responsible for that runs
before mine.