RFR: Minor cleanup and refactoring

[Ashutosh Mehra]

Removed unused parameter `aot_code_entry` in `ciEnv::register_method`.
Removed `preload` local variable in `ciEnv::register_method` as it is always false.
Refactored `ciEnv::is_compilation_valid` a bit.

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Commit messages:
 - Minor cleanup and refactoring

Changes: https://git.openjdk.org/leyden/pull/101/files
  Webrev: https://webrevs.openjdk.org/?repo=leyden&pr=101&range=00
  Stats: 45 lines in 5 files changed: 6 ins; 24 del; 15 mod
  Patch: https://git.openjdk.org/leyden/pull/101.diff
  Fetch: git fetch https://git.openjdk.org/leyden.git pull/101/head:pull/101

PR: https://git.openjdk.org/leyden/pull/101