[Flang][OpenMP] compilation failed with "Basic Block in function '_QQmain' does not have terminator!"

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Reproducible on Godbolt:
https://godbolt.org/z/zMjo7ajYv

!$omp parallel
!$omp cancel parallel
!$omp barrier
!$omp end parallel
end

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With assertion-enabled flang, the following output is obtained:

flang-21: warning: OpenMP support in flang is still experimental [-Wexperimental-option]
Basic Block in function '_QQmain' does not have terminator!
label %omp.par.region1.split.cont
While deleting: label %omp.par.region1.split.cont
Use still stuck around after Def is destroyed:  br i1 %5, label %omp.par.region1.split.cont, label %omp.par.region1.split.cncl
Uses remain when a value is destroyed!
UNREACHABLE executed at /path_to_project/llvm-project/llvm/lib/IR/Value.cpp:102!