RFR: type of TypeVar.ref is incorrect and should be fixed

[Vicente Romero]

The type of TypeVar.ref is incorrect and should be fixed, in particular due to a bug, the compiler is not able to differentiate between  `T` and `T.ref`. Also did some cleanup to warnings related code, in particular there should be a different warning for cases when a universal tvar is not assigned `null` but it is assigned a value that could be null at run time. This warning should not be the same as the one for a plain null assignment.

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Commit messages:
 - clean up to warnings related code

Changes: https://git.openjdk.java.net/valhalla/pull/658/files
 Webrev: https://webrevs.openjdk.java.net/?repo=valhalla&pr=658&range=00
  Stats: 87 lines in 6 files changed: 77 ins; 5 del; 5 mod
  Patch: https://git.openjdk.java.net/valhalla/pull/658.diff
  Fetch: git fetch https://git.openjdk.java.net/valhalla pull/658/head:pull/658

PR: https://git.openjdk.java.net/valhalla/pull/658