RFR: addressing several issues in the implementation of universal tvars [v5]

[Vicente Romero]

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

Vicente Romero has updated the pull request incrementally with one additional commit since the last revision:

  missed a case in unchecked conversions

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Changes:
  - all: https://git.openjdk.java.net/valhalla/pull/684/files
  - new: https://git.openjdk.java.net/valhalla/pull/684/files/a3cc93a2..d005964b

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=valhalla&pr=684&range=04
 - incr: https://webrevs.openjdk.java.net/?repo=valhalla&pr=684&range=03-04

  Stats: 25 lines in 2 files changed: 21 ins; 0 del; 4 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