[SYCL-MLIR] Opaque pointer support in SYCL-to-LLVM conversion

[sommerlukas]

First step to add opaque pointer support for the SYCL-MLIR project: Make the lowering patterns in the SYCL-to-LLVM lowering compatible with opaque pointers.

Emitting typed or opaque pointers is controlled by the use-opaque-pointers pass option, that was added.

Partly resolves #8616.

[sommerlukas]

Similar to #8767, the existing tests were copied to a file with a -typed-pointer suffix and the original test files were updated to test opaque pointers.

In contrast to #8767, I did not duplicate the patterns, but rather added logic for opaque and typed pointers where necessary.

[victor-eds]

For tests with opaque pointers, can we have the full line checked?, i.e., add the missing types after the -> !llvm.ptr in GEPOp, loaded types in LoadOp, etc.

[victor-eds]

Just a single NIT. LGTM if you agree or not.