[Unity][Transform] Raise error in FuseOpsByPattern for SSA violation

[Lunderberg]

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Internally, FuseOpsByPattern makes a mapping from relax variables to the fused group containing that variable. If the input module violates SSA, this map may be ill-formed. While not strictly necessary for FuseOps to handle ill-formed inputs, checking it at this level provides better error handling than propagating it to downstream passes.

This commit checks for ill-formed inputs that would produce invalid fused outputs and raises an error.

[Lunderberg]

Rebased onto main to resolve conflicts