[mlir][Vector] Add pattern to reorder elementwise and broadcast ops

The new pattern will replace elementwise(broadcast) with
broadcast(elementwise) when safe.

This change affects tests for vectorising nD-extract. In one case
("vectorize_nd_tensor_extract_with_tensor_extract") I just trimmed the
test and only preserved the key parts (scalar and contiguous load from
the original Op). We could do the same with some other tests if that
helps maintainability.

Differential Revision: https://reviews.llvm.org/D152812