[MLIR][XeGPU] Clean up helpers in XeGPUPropagateLayout (#175857)

In XeGPUPropagateLayout.cpp, the helper getDefaultSIMTLayoutInfo is
implemented via multiple overloads that differ significantly in
semantics, not just parameter types.
Reusing the same function name for these semantically different
behaviors makes call sites harder to read and reason about and increases
the maintenance burden. This PR improves readability and maintainability
of layout propagation logic.

GitOrigin-RevId: 3150b73dec83d76321727a3c01a7907485bec6ce