RFR: addressing several issues in the implementation of universal tvars

[Vicente Romero]

Last review iteration found several issues in the current implementation, addressing them here

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Commit messages:
 - compiler is issuing multiple warnings for the same issue

Changes: https://git.openjdk.java.net/valhalla/pull/684/files
 Webrev: https://webrevs.openjdk.java.net/?repo=valhalla&pr=684&range=00
  Stats: 24 lines in 3 files changed: 24 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/valhalla/pull/684.diff
  Fetch: git fetch https://git.openjdk.java.net/valhalla pull/684/head:pull/684

PR: https://git.openjdk.java.net/valhalla/pull/684