RFR: 8288628: Unnecessary Hashtable usage in ConditionalSpecialCasing

[Andrey Turbanov]

If a thread-safe implementation is not needed, it is recommended to use HashMap in place of Hashtable.
`ConditionalSpecialCasing.entryTable` is read-only Map which is modified only in `static` block. It means we can safely replace it with HashMap.

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Commit messages:
 - [PATCH] Unnecessary Hashtable usage in ConditionalSpecialCasing

Changes: https://git.openjdk.org/jdk/pull/9172/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9172&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8288628
  Stats: 4 lines in 1 file changed: 1 ins; 1 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/9172.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9172/head:pull/9172

PR: https://git.openjdk.org/jdk/pull/9172