MPI nodes don't handle scalar args correctly

**Describe the bug**
The dace program fails to compile if the integer args of MPI nodes (`dst` and `tag`) are anything other than symbols, symbol expressions and numbers.

**To Reproduce**
```python
rank = dc.symbol('rank', dtype=dc.int64)

@dc.program
def func(A: dc.int32[N]):
# dace.comm.Send(A[0], rank - 1, 0) # Works
dace.comm.Send(A[0], abs(rank - 1), 0)
```
The program fails to compile with the following error:
```
ValueError: Node type "Send" not supported for promotion
```

Same behavior in other scenarios:
```python
# ...
a = 0
a = rank
a = A[0]
dace.comm.Send(A[0], a, 0)
```

**Desktop (please complete the following information):**
- Latest DaCe master branch

**Possible fix:**
The code below should check if the `sdfg.arrays` entry for the corresponding arg is a `Scalar` when given an `str` and fall to the last branch.

https://github.com/spcl/dace/blob/f4b4d01f67cb089b3ef821673e0a12405c94f9b1/dace/frontend/common/distr.py#L424-L436

Start with dace/frontend/common/distr.py around lines 424-436 and reproduce the failure using the provided dc.program example with abs(rank - 1) and scalar assignments. Verify the MPI Send and related node cases compile when dst or tag receives scalar expressions, symbols, numbers, or array elements, while existing working cases remain valid.

Written by the indexing model from the issue text.