[Flang][OpenMP] Referring to an instruction in another function!

Reproducible on Godbolt:
https://godbolt.org/z/44Ej9E6Gj
```f90
integer, parameter :: N = 100000, CHUNKSZ = 10000
real :: Z(N)
integer :: C, i

do C = 1, N, CHUNKSZ
    !$omp task
        !$omp target map(Z(C:C+CHUNKSZ-1))
            !$omp parallel do
            do i = C, C+CHUNKSZ-1
            end do
        !$omp end target
    !$omp end task
end do
end
```