r1.12-rc1 cherry-pick request: Make sure that all operands and outputs of Sort...

r1.12-rc1 cherry-pick request: Make sure that all operands and outputs of Sort have the same layout. (#22922)

* [XLA] Migrate from gtl::FlatSet to absl::flat_hash_set

PiperOrigin-RevId: 215324035

* [XLA] Modify the function that determines whether an instruction can change
layout so that it can be used by the HLO verifier.

Change the function to a static member function of the LayoutAssignment class.

Add an std::function member to LayoutAssignment to store the function object
passed down from the backend compiler class and use it to decide whether an
instruction can change layouts.

Fix affected test cases.

PiperOrigin-RevId: 215515611

* Make sure that all operands and outputs of Sort have the same layout.

Also fix the DotLayout test, it would pass even when commenting out the dot specific logic in GpuLayoutAssignment.

PiperOrigin-RevId: 216726796