[mlir][linalg] Fold tensor.pad when inserting into linalg.fill

Fold tensor.insert_slice(tensor.pad(<input>), linalg.fill) into
tensor.insert_slice(<input>, linalg.fill) if the padding value and
the filling value are the same.

Reviewed By: ThomasRaoux

Differential Revision: https://reviews.llvm.org/D120410