RFR: 8258048: Placeholder hash code is the same as Dictionary hash code

[Coleen Phillimore]

This change is a set of cleanups to the placeholders hashtable.  It moves the index calculation inside the hashtable.  It avoids using the placeholder hash code because it's the same hashcode as the dictionary hashcode.  There's are some assert for this.

One of the find_class() functions is redundant to dictionary->find_class() so is removed.

Tested with tier1-6.

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Commit messages:
 - 8258048: Placeholder hash code is the same as Dictionary hash code

Changes: https://git.openjdk.java.net/jdk/pull/1789/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1789&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8258048
  Stats: 129 lines in 8 files changed: 6 ins; 31 del; 92 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1789.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1789/head:pull/1789

PR: https://git.openjdk.java.net/jdk/pull/1789