[mlir][vector] Allow out-of-bounds starting positition for vector transfer ops

The starting indices of all vector dimensions are allowed to be out-of-bounds.

E.g.:
```
// %j is allowed to be out-of-bounds (but not %i).
%0 = vector.transfer_read %m[%i, %j] ... {in_bounds = [false]} : memref<?x?xf32>, vector<5xf32>
```

This revision just updates the op documentation and adds extra test cases. Out-of-bounds starting points are already supported by the respective lowerings:
* 2D and higher-dimensional transfers are lowered to 1D transfers by `VectorToScf`. These patterns generate an `scf.if` check for every (potentially unrolled) loop iteration if the dimension is `in_bounds = false`, including the first loop iteration.
- 1D out-of-bounds transfers are lowered to in-bounds transfers by `MaterializeTransferMask`, which adds a mask to the op. The mask is defined by `vector.create_mask (dim-size) - (index)`. In case of an out-of-bounds starting point, the operand of the `vector.create_mask` op is 0 or negative. Negative operands are treated like 0 according to the documentation of `vector.create_mask`.

Differential Revision: https://reviews.llvm.org/D155719