fix(tmt): Fix handling of duplicate output values in TosaSerialize

This commit fixes a bug which invalidated a mapping of mlir result operands to serialization_lib output names. When a duplicate intermediate operand was provided, a new output name would be created, rather than using the existing output name in the mapping. Now, if the operand is already found in the return_values mapping, its existing name is reused.

Signed-off-by: Luke Hutton luke.hutton@arm.com