RFR: 8304743: Compile_lock and SystemDictionary updates [v3]

[Coleen Phillimore]

> The SystemDictionary is updated and read under the Compile_lock but this is unnecessary because the table is a concurrent hashtable, and the lock doesn't really synchronize any other compilation state.  The lock may have protected other state that Dependencies used in the past but has been removed.   See discussion in CR for more information.
> Tested with tier1-8.

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

  Missing null check

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/13270/files
  - new: https://git.openjdk.org/jdk/pull/13270/files/0d48d2a8..a53ab524

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13270&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13270&range=01-02

  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/13270.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13270/head:pull/13270

PR: https://git.openjdk.org/jdk/pull/13270