[mlir][vector] Untangle TransferWriteDistribution and avoid crashing in the 0-D case.

This revision avoids a crash in the 0-D case of distributing vector.transfer ops out of
vector.warp_execute_on_lane_0.
Due to the code complexity and lack of documentation, it took untangling the implementation
before realizing that the simple fix was to fail in the 0-D case.
The rewrite is still very useful to understand this code better.

Differential Revision: https://reviews.llvm.org/D128793