[mlir][crunner] fix bug in memref copy for rank 0

While replacing linalg.copy with the more desired memref.copy
I found a bug in the support library for rank 0 memref copying.
The code would loop for something like the following, since there
is code for no-rank and rank > 0, but rank == 0 was unexpected.

  memref.copy %0, %1: memref<f32> to memref<f32>

Note that a "regression test" for this will follow using the
sparse compiler migration to memref.copy which exercises this
case many times.

Reviewed By: herhut

Differential Revision: https://reviews.llvm.org/D106036