RFR: 8253636: C2: Adjust NodeClasses::_max_classes

[Roberto Castañeda Lozano]

Update `NodeClasses::_max_classes` to the max class id within the enumeration. Update comment and assertion to reflect
that `NodeClasses` uses now 32 bits after the addition of `Opaque1` in JDK-8229495.

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

Commit messages:
 - 8253636: C2: Adjust NodeClasses::_max_classes

Changes: https://git.openjdk.java.net/jdk/pull/397/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=397&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8253636
  Stats: 3 lines in 2 files changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/397.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/397/head:pull/397

PR: https://git.openjdk.java.net/jdk/pull/397