[mlir][vector] Take dim sizes into account in DropInnerMostUnitDims. (#71752)

The `stride == 1` does not imply that we can drop it. Because it could
load more than 1 elements. We should also take source sizes and vector
sizes into account. Otherwise it generates invalid IRs. E.g.,

```mlir
func.func @foo(%arg0: memref<1x1xf32>) -> vector<4x8xf32> {
  %c0 = arith.constant 0 : index
  %cst = arith.constant 0.000000e+00 : f32
  %0 = vector.transfer_read %arg0[%c0, %c0], %cst : memref<1x1xf32>, vector<4x8xf32>
  return %0 : vector<4x8xf32>
}
```

Fixes https://github.com/openxla/iree/issues/15493