[mlir] Don't emit unused labels

Stop the Cpp target from emitting unused labels. The previosly generated
code generated warning if `-Wunused-label` is passed to a compiler.

Co-authored-by: Simon Camphausen <simon.camphausen@iml.fraunhofer.de>

Reviewed By: jpienaar

Differential Revision: https://reviews.llvm.org/D118154