IR Refactor

[wingertge]

Refactors IR to make it more ergonomic by separating the output of an operation from the operation itself, and separating the type of any given variable from the kind/identifier.
Moves builtin variables into a separate enum so they no longer fill the VariableKind enum with a bunch of semantically identical duplicates. This is more in line with other variable kinds that use IDs to indentify the specific variable instance.
Moves atomic operations to their own enum.
Moves Select from Branch to Operation.
Adds explicit Cast operator to avoid confusing implicit casting on Assign.
Makes Assign a first class Operation since now that Cast is explicit it's essentially a register copy and thus has different semantics compared to other operations (for example, it could be safely inlined and removed without affecting execution).

There's a lot of churn but it's overall cleaner, and actually lowers LOC by almost 400. It also simplifies any future optimization passes that may be added to cubecl-opt. Things that only care about the output can now access it directly, and vice versa for the operation and inputs.

Testing

All tests pass in both burn and cubecl. I've already prepared a burn version where all legacy kernels are migrated to the refactored variables.

[nathanielsimard]

Maybe Identity is better here? Assign without an output is a bit strange.

[wingertge]

Normally it would be called copy but that was already used. Maybe we should just rename Copy to CopyMemory (which is in line with the SPIR-V name and memcpy), and call this one Copy. Because that's also how it's still used: to make a copy of a mutable variable.