[flang] [llvm] [flang][OpenMP] Decompose compound constructs, do recursive lowering (PR #90098)

[Pete Steinfeld via llvm-commits]

psteinfeld wrote:

@kparzysz, this change is causing one of our internal tests to fail.  Here's an excerpt from the log file:
```
flang -I. -I. -c -mp -O3 -DNO_GPU -DNO_AMD -DNO_PHI -DLINUX -DNO_GNU -DNO_PGI -fpic /proj/ta/tests/applications/cpu/GFC/src/stsubs.F90 -o ./OBJ/stsubs.o
f18: /proj/build/llvm/Linux_x86_64/flang/lib/Lower/OpenMP/Decomposer.cpp:96: Fortran::lower::omp::ConstructQueue Fortran::lower::omp::buildConstructQueue(mlir::ModuleOp, Fortran::semantics::SemanticsContext&, Fortran::lower::pft::Evaluation&, const Fortran::parser::CharBlock&, llvm::omp::Directive, Fortran::lower::omp::List<Fortran::lower::omp::Clause>&): Assertion `!decompose.output.empty() && "Construct decomposition failed"' failed.
make[1]: *** [/proj/ta/tests/applications/cpu/GFC/src/Makefile:216: OBJ/stsubs.o] Aborted (core dumped)
make[1]: Leaving directory '/tmp/qa/GFC_0'
make: *** [/proj/ta/tests/applications/cpu/GFC/makefile:15: build] Error 2
make TOOLKIT=LLVM SRC=/proj/ta/tests/applications/cpu/GFC/src -f /proj/ta/tests/applications/cpu/GFC/src/Makefile
make[1]: Entering directory '/tmp/qa/GFC_0'
flang -I. -I. -c -mp -O3 -DNO_GPU -DNO_AMD -DNO_PHI -DLINUX -DNO_GNU -DNO_PGI -fpic /proj/ta/tests/applications/cpu/GFC/src/stsubs.F90 -o ./OBJ/stsubs.o
f18: /proj/build/llvm/Linux_x86_64/flang/lib/Lower/OpenMP/Decomposer.cpp:96: Fortran::lower::omp::ConstructQueue Fortran::lower::omp::buildConstructQueue(mlir::ModuleOp, Fortran::semantics::SemanticsContext&, Fortran::lower::pft::Evaluation&, const Fortran::parser::CharBlock&, llvm::omp::Directive, Fortran::lower::omp::List<Fortran::lower::omp::Clause>&): Assertion `!decompose.output.empty() && "Construct decomposition failed"' failed.
make[1]: *** [/proj/ta/tests/applications/cpu/GFC/src/Makefile:216: OBJ/stsubs.o] Aborted (core dumped)
```
I'll work on getting a small reproducer.

https://github.com/llvm/llvm-project/pull/90098