[mlir][Linalg] Pattern to fuse pad operation with elementwise operations.

Most convolution operations need explicit padding of the input to
ensure all accesses are inbounds. In such cases, having a pad
operation can be a significant overhead. One way to reduce that
overhead is to try to fuse the pad operation with the producer of its
source.

A sequence

```
linalg.generic -> linalg.pad_tensor
```

can be replaced with

```
linalg.fill -> tensor.extract_slice -> linalg.generic ->
tensor.insert_slice.
```

if the `linalg.generic` has all parallel iterator types.

Differential Revision: https://reviews.llvm.org/D116418