[mlir][linalg] Transform PadTensorOp into InitOp, FillOp, GenericOp

Introduces a test pass that rewrites PadTensorOps with static shapes as a sequence of:

```
linalg.init_tensor // to create output
linalg.fill        // to initialize with padding value
linalg.generic     // to copy the original contents to the padded tensor
```

The pass can be triggered with:

- `--test-linalg-transform-patterns="test-transform-pad-tensor"`

Differential Revision: https://reviews.llvm.org/D102804