[mlir][cf] Add support for opaque pointers to ControlFlowToLLVM lowering

Part of https://discourse.llvm.org/t/rfc-switching-the-llvm-dialect-and-dialect-lowerings-to-opaque-pointers/68179

This is a very simple patch since there is only one use of pointers types in `cf.assert` that has to be changed. Pointer types are conditionally created with element types and the GEP had to be adjusted to use the array type as base type.

Differential Revision: https://reviews.llvm.org/D143583