RFR: 8258048: Placeholder hash code is the same as Dictionary hash code [v3]

[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 hashcode because it's the same hashcode as the dictionary hashcode.  There are some asserts for this.
> 
> One of the find_class() functions is redundant to dictionary->find_class() so is removed.
> 
> Tested with tier1-6.

Coleen Phillimore has updated the pull request incrementally with one additional commit since the last revision:

  Fix or remove extraneous null checks.

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/1789/files
  - new: https://git.openjdk.java.net/jdk/pull/1789/files/ddf96fa2..0ec6dc91

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=1789&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=1789&range=01-02

  Stats: 5 lines in 1 file changed: 0 ins; 1 del; 4 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