[mlir][vector] Add support for unrolling vector.bitcast ops. (#94064)

The revision unrolls vector.bitcast like:

```mlir
%0 = vector.bitcast %arg0 : vector<2x4xi32> to vector<2x2xi64>
```

to

```mlir
%cst = arith.constant dense<0> : vector<2x2xi64>
%0 = vector.extract %arg0[0] : vector<4xi32> from vector<2x4xi32>
%1 = vector.bitcast %0 : vector<4xi32> to vector<2xi64>
%2 = vector.insert %1, %cst [0] : vector<2xi64> into vector<2x2xi64>
%3 = vector.extract %arg0[1] : vector<4xi32> from vector<2x4xi32>
%4 = vector.bitcast %3 : vector<4xi32> to vector<2xi64>
%5 = vector.insert %4, %2 [1] : vector<2xi64> into vector<2x2xi64>
```

The scalable vector is not supported because of the limitation of
`vector::createUnrollIterator`. The targetRank could mismatch the final
rank during unrolling; there is no direct way to query what the final
rank is from the object.