[mlir][ArmSVE] Add convert_to/from_svbool ops (#68586)

This adds slightly higher-level ops for converting masks between svbool
and SVE predicate types. The main reason to use these over the
intrinsics is these ops support vectors of masks (via unrolling).

E.g.

```
// Convert a svbool mask to a mask of SVE predicates:
%svbool = vector.load %memref[%c0, %c0]
                       : memref<2x?xi1>, vector<2x[16]xi1>
%mask = arm_sve.convert_from_svbool %svbool : vector<2x[8]xi1>
// => Results in vector<2x[8]xi1>
```
Or:
```
// Convert a mask of SVE predicates to a svbool mask:
%mask = vector.create_mask %c2, %dim_size : vector<2x[2]xi1>
%svbool = arm_sve.convert_to_svbool %mask : vector<2x[2]xi1>
// => Results in vector<2x[16]xi1>
```

Depends on #68418