[code-reflection] RFR: Fix modeling if statements

[Paul Sandoz]

The else part of an if statement was incorrectly handled if it was not a block.

Additionally fixed `CodeReflectionTester` used in the language tests. It now accesses a code model of a method via getCodeModel rather than directly accessing the field, thereby this is not dependent on how the model is stored in the class file.

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Commit messages:
 - Removed unused local variable
 - Fix modeling if statements.

Changes: https://git.openjdk.org/babylon/pull/181/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=181&range=00
  Stats: 111 lines in 3 files changed: 103 ins; 5 del; 3 mod
  Patch: https://git.openjdk.org/babylon/pull/181.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/181/head:pull/181

PR: https://git.openjdk.org/babylon/pull/181