[flang][OpenMP] Lowering crashes for composite do simd constructs with private clause

[mrkajetanp]

After recent changes to the lowering of composite do simd constructs, programs like the following no longer ignore the simd information.

program main
  integer, allocatable :: tmp
  allocate(tmp)
  !$omp do simd private(tmp)
  do i=1, 10
    tmp = tmp + 1
  end do
end program

Instead, they crash with:

error: loc("/repr.f90":4:9): 'omp.wsloop' op loop wrapper does not contain exactly one nested op
error: verification of lowering to FIR failed

[llvmbot]

@llvm/issue-subscribers-flang-ir

Author: Kajetan Puchalski (mrkajetanp)

After recent changes to the lowering of composite do simd constructs, programs like the following no longer ignore the simd information. ``` program main integer, allocatable :: tmp allocate(tmp) !$omp do simd private(tmp) do i=1, 10 tmp = tmp + 1 end do end program ``` Instead, they crash with: ``` error: loc("/repr.f90":4:9): 'omp.wsloop' op loop wrapper does not contain exactly one nested op error: verification of lowering to FIR failed ```