Generate nuw GEPs for struct member accesses, as inbounds + non-negative
implies nuw.
Regression tests are updated using update scripts where possible, and by
find + replace where not.
Add the reverse directive which will be introduced in the upcoming
OpenMP 6.0 specification. A preview has been published in [Technical
Report 12](https://www.openmp.org/wp-content/uploads/openmp-TR12.pdf).
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Co-authored-by: Alexey Bataev <a.bataev@outlook.com>