[jdk21u-dev] RFR: 8340024: In ClassReader, extract a constant for the superclass supertype_index

[Severin Gehwolf]

Simple non-functional change. Clean-up as discussed in https://github.com/openjdk/jdk21u-dev/pull/1251

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Commit messages:
 - Backport 68c4f36857a8ce62731cc73e251e969d48e526ef

Changes: https://git.openjdk.org/jdk21u-dev/pull/1671/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=1671&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8340024
  Stats: 7 lines in 1 file changed: 6 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/1671.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/1671/head:pull/1671

PR: https://git.openjdk.org/jdk21u-dev/pull/1671