[mlir][vector] Fix invalid `LoadOp` indices being created (#75519)

Fixes https://github.com/llvm/llvm-project/issues/71326.

The cause of the issue was that a new `LoadOp` was created which looked
something like:
```mlir
%arg4 = 
func.func main(%arg1 : index, %arg2 : index) {
  %alloca_0 = memref.alloca() : memref<vector<1x32xi1>>
  %1 = vector.type_cast %alloca_0 : memref<vector<1x32xi1>> to memref<1xvector<32xi1>>
  %2 = memref.load %1[%arg1, %arg2] : memref<1xvector<32xi1>>
  return
}
```
which crashed inside the `LoadOp::verify`. Note here that `%alloca_0` is
0 dimensional, `%1` has one dimension, but `memref.load` tries to index
`%1` with two indices.

This is now fixed by using the fact that `unpackOneDim` always unpacks
one dim


https://github.com/llvm/llvm-project/blob/1bce61e6b01b38e04260be4f422bbae59c34c766/mlir/lib/Conversion/VectorToSCF/VectorToSCF.cpp#L897-L903



and so the `loadOp` should just index only one dimension.

---------

Co-authored-by: Benjamin Maxwell <macdue@dueutil.tech>