[clang] [llvm] [clang][OpenMP] Improve loop structure for distributed loops (PR #201670)

[Robert Imschweiler via llvm-commits]

ro-i wrote:

One would think that the changed loop structure is basically a no-op for non-reduction loops. But there seem to be some downstream optimizations (maybe unrolling the inner loop).
The one-line change easy way out is to just gate my loop change so that it takes only effect for reduction loops. But give me a moment to test sth I'm thinking about

https://github.com/llvm/llvm-project/pull/201670