[mlir][arith] Refine the verifier for arith.constant (#86178)

Disallows initialization of scalable vectors with an attribute of
arbitrary values, e.g.:
```mlir
  %c = arith.constant dense<[0, 1]> : vector<[2] x i32>
```

Initialization using vector splats remains allowed (i.e. when all the
init values are identical):
```mlir
  %c = arith.constant dense<[1, 1]> : vector<[2] x i32>
```