[mlir][vector] Don't treat memrefs with empty stride as non-contiguous (#76848)

As per the docs [1]:

```
In absence of an explicit layout, a memref is considered to have a
multi-dimensional identity affine map layout.
```

This patch makes sure that MemRefs with no strides (i.e. no explicit
layout) are treated as contiguous when checking whether a particular
vector is a contiguous slice of the given MemRef.

[1] https://mlir.llvm.org/docs/Dialects/Builtin/#layout

Follow-up for #76428.