[GPU] XeVM to LLVM pass structure

[kurapov-peter]

This adds the lowering pass boilerplate for convert-xevm-to-llvm.

Depends on #390.

[dchigarev]

We might also want to move some of the passes from 'Transforms' to 'Conversion' at some point in the future, right? (linalg-to-xegpu, gpu-to-gpuocl, lower-to-tile-vector, convert-memref-to-cpuruntime, ...)

[kurapov-peter]

We might also want to move some of the passes from 'Transforms' to 'Conversion' at some point in the future, right? (linalg-to-xegpu, gpu-to-gpuocl, lower-to-tile-vector, convert-memref-to-cpuruntime, ...)

Yes, some of them are conversions, not transforms.