[flang] [openmp] Flang with OMP 5.1 accepts Capture and Expected variables to be the same in an atomic compare capture

Release builds of Flang with OMP 5.1 currently compile the following without error:

```fortran
subroutine pre_cap_02(var1, num1, num2, num3)
integer :: var1, num1, num2, num3
!$omp atomic compare capture
num1 = num1
if (var1 == num1) then
var1 = num2
end if
!$omp end atomic
end subroutine
```

Capture and Expected variables cannot be the same in an OpenMP atomic compare capture region.

Debug builds of Flang with OMP 5.1 with the same code, fail to compile with a failed assert in `OmpStructureChecker::CheckAtomicCaptureAssignment` within `flang/lib/Semantics/check-omp-atomic.cpp` at lines 841-843. That assert was recently changed, so I suspect this is a simple oversight.

Start in flang/lib/Semantics/check-omp-atomic.cpp at OmpStructureChecker::CheckAtomicCaptureAssignment, especially the logic around lines 841-843. Reproduce the supplied atomic compare capture example in release and debug builds. Done means the invalid shared Capture and Expected variables are rejected consistently without a failed assertion.

Written by the indexing model from the issue text.