[llvm] [mlir] [OMPIRBuilder] - Fix emitTargetTaskProxyFunc to not generate empty functions (PR #126958)

[Sergio Afonso via llvm-commits]

https://github.com/skatrak approved this pull request.

LGTM, just minor nits from me. I also agree with @ergawy about using `splitBB` to simplify the implementation.

https://github.com/llvm/llvm-project/pull/126958