[mlir] Handle cases where transfer_read should turn into a scalar load

The existing vector transforms reduce the dimension of transfer_read
ops.  However, beyond a certain point, the vector op actually has
to be reduced to a scalar load, since we can't load a zero-dimension
vector.  This handles this case.

Note that in the longer term, it may be preferaby to support
zero-dimension vectors.  see
https://llvm.discourse.group/t/should-we-have-0-d-vectors/3097.

Differential Revision: https://reviews.llvm.org/D103432