[Flang] Hang / infinite loop regression in OpenMP tests (in last ~2 weeks)

[mgorny]

While retesting Flang last Saturday, I've noticed a few tests were hanging. Today I've rechecked it, and they still are.

The hanging tests are:

  Flang :: Integration/OpenMP/atomic-capture-complex.f90
  Flang :: Lower/OpenMP/atomic-implicit-cast.f90
  Flang :: Lower/OpenMP/atomic-privatize.f90
  Flang :: Lower/OpenMP/common-atomic-lowering.f90
  Flang :: Lower/OpenMP/dump-atomic-analysis.f90

This is Gentoo amd64 standalone build. Confirmed with 4373463 and c5972da. I'm going to try bisecting but it's going to take a fair while.

[mgorny]

The last known good was 814ac2e but I need to recheck it, in case it was something external breaking it.