Fix handling of rank-1 tensors in tosa.reduce_sum

The conversion of `tosa.reduce_sum` to linalg creates a
`linalg.generic` op that produces a tensor of rank `input_rank -
1`. This tensor is then expanded back into a tensor of rank
`input_rank`. In the case where the tensor being expanded is rank-0,
the reassociation map used must be empty. However, the current
implementation indexes and modifies the reassociation map independent
of the rank of the tensor being expanded, resulting in out-of-bounds
indexing when the tensor being expanded is rank-0. This commit adds a
guard to the reassociation map indexing.

Reviewed By: jpienaar

Differential Revision: https://reviews.llvm.org/D135828