[llvm] [mlir] [OpenMP][OMPIRBuilder] Fix lastiter assertion in target workshare loop (PR #214996)

[via llvm-commits]

RohithPariki wrote:

> This diff has no test. Please add a test using the reproducer.

Thanks for the thorough review!

1. **Race condition fix**: you are completely right. Forcing `lastiter` to 1 caused all threads executing the outer kernel to take the write-back branch. I've updated `applyWorkshareLoopTarget` to initialize `p.lastiter` to `0` and evaluate whether the current iteration counter inside the outlined loop body equals `TripCount - 1`. `p.lastiter` is updated to `1` only during the last iteration, so only the thread executing the last iteration will perform the write-back.

2. **Added Test**: i have added a test in `mlir/test/Target/LLVMIR/openmp-target-wsloop-linear.mlir` using the reproducer from #213905 to verify both `lastiter` initialization and the in-body last iteration check during LLVM IR lowering.


https://github.com/llvm/llvm-project/pull/214996