[code-reflection] RFR: Uninitialized variables

[Paul Sandoz]

Model uninitialized variables as a var operation with no operand.

This is much simpler than #263

-------------

Commit messages:
 - Clean ups
 - Update comment
 - Remove unknown value op and type
 - Uninitialized variable modeled as var with no operand
 - Typo
 - Cleanups
 - Test for uninitialized variable
 - Naming updates
 - Use the term unknown rather than undefined
 - Model unitialized variables

Changes: https://git.openjdk.org/babylon/pull/264/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=264&range=00
  Stats: 209 lines in 10 files changed: 175 ins; 10 del; 24 mod
  Patch: https://git.openjdk.org/babylon/pull/264.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/264/head:pull/264

PR: https://git.openjdk.org/babylon/pull/264