[OpenMP] Optimized trivial multiple edges from task dependency graph

From "3.1 Reducing the number of edges" of this [[ https://hal.science/hal-04136674v1/ | paper ]] - Optimization (b)

Task (depen

[OpenMP] Optimized trivial multiple edges from task dependency graph

From "3.1 Reducing the number of edges" of this [[ https://hal.science/hal-04136674v1/ | paper ]] - Optimization (b)

Task (dependency) nodes have a `successors` list built upon passed dependency.
Given the following code, B will be added to A's successors list building the graph `A` -> `B`
```
// A
# pragma omp task depend(out: x)
{}

// B
# pragma omp task depend(in: x)
{}
```

In the following code, B is currently added twice to A's successor list
```
// A
# pragma omp task depend(out: x, y)
{}

// B
# pragma omp task depend(in: x, y)
{}
```

This patch removes such dupplicates by checking lastly inserted task in `A` successor list.

Authored by: Romain Pereira (rpereira-dev)
Differential Revision: https://reviews.llvm.org/D158544

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