[llvm] [slp] fix scheduler deadlock for copyable users of vectorized instructions (PR #176661)

[via llvm-commits]

Serosh-commits wrote:

 see now that the previous approach was indeed just masking the symptom by forcing recalculations.
i have  refactored the patch to address the root cause in the dependency model. The deadlock occurred because the scheduler was tracking dependencies on anchored(non-schedulable) parent nodes. Since these fixed nodes never 'schedule,' they were permanently blocking their operands.
I've updated -
calculateDependencies.cpp
 to skip incrementing unscheduled dependencies for any user node that 
doesNotNeedToSchedule.cpp
This correctly models the graph by acknowledging that these fixed anchors don't participate in the movement-based scheduling of their operands

https://github.com/llvm/llvm-project/pull/176661