[llvm] [ScheduleDAG] Fix and simplify the algorithm used for finding callseq_start (PR #149692)

[Sergei Barannikov via llvm-commits]

s-barannikov wrote:

FWIW CALLSEQ_START/CALLSEQ_END cannot be nested. This is [enforced](https://github.com/llvm/llvm-project/blob/4c27279ec4c255b93a4cb3f044509121ebf01f10/llvm/lib/CodeGen/MachineVerifier.cpp#L3992) by MachineVerifier.


https://github.com/llvm/llvm-project/pull/149692