[XLA:CPU] Allow the shape partition algorithm to partition the most minor

dimension.

The current shape paritition algorithm does not partition the most minor
dimension, because doing so causes dynamic loop bounds for the inner loop and
used to prohibit LLVM vectorization. This constraint has been removed with
revision 328478 and LLVM can now vectorize loops with dynamic bounds. Allow
partitioning the most minor dimension is also necessary to support the
parallelization of matrix-vector multiplication.

Adjust shape_partition_test to reflect this change in the shape partition
algorithm.

PiperOrigin-RevId: 190574615