RFR: 8279876: Clean up: isAssignableFrom usages in xpath and jdk internal classes

[Joe Wang]

Clean up the usages of isAssignableFrom in a few xpath and jdk/internal classes where the checks were really about equality or whether they were the exact class types. It was why they worked nonetheless even though some of them were backwards.

Test: existing tests passed.

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Commit messages:
 - 8279876: Clean up: isAssignableFrom usages in xpath and jdk internal classes

Changes: https://git.openjdk.java.net/jdk/pull/8116/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8116&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8279876
  Stats: 26 lines in 6 files changed: 1 ins; 0 del; 25 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8116.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8116/head:pull/8116

PR: https://git.openjdk.java.net/jdk/pull/8116