RFR: 8287064: Modernize ProxyGenerator.PrimitiveTypeInfo

[liach]

Simplify opcode handling, use `final` in `PrimitiveTypeInfo`, and replace the hash map with a simple lookup, similar to what's done in [JDK-8284880](https://bugs.openjdk.java.net/browse/JDK-8284880) (#8242)

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Commit messages:
 - 8287064: Modernize ProxyGenerator.PrimitiveTypeInfo

Changes: https://git.openjdk.java.net/jdk/pull/8801/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8801&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8287064
  Stats: 79 lines in 1 file changed: 14 ins; 41 del; 24 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8801.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8801/head:pull/8801

PR: https://git.openjdk.java.net/jdk/pull/8801