Update Ptx constant-proxy rule

[tonghaining]

This PR contains 2 updates:

1. As verified in PTX document, weak instructions don't hold the scope tag. It is removed from the visitors now.
2. We found the addition of the constant-proxy rule will no longer be necessary following the modification that explicitly defines locations as Constant virtually aliasing to Generic locations. There is a corresponding pull request toward the original PTX alloy approach. We could await confirmation on whether it is appropriate to proceed with this change.

[hernanponcedeleon]

There has not been any answer regarding the PR to the repository with the alloy model. However, the model in this PR is more faithful to their model, so I will merge.