Mark TensorDialect legal and PadTensor op illegal

`GeneralizePadTensorOpPattern` might generate `tensor.dim` op so the
TensorDialect should be marked legal. This pattern should also
transform all `linalg.pad_tensor` ops so mark those as illegal. Those
changes are missed from a previous change in
https://reviews.llvm.org/D105293

Reviewed By: silvas

Differential Revision: https://reviews.llvm.org/D105642