RFR: 8279876: Clean up: isAssignableFrom usages in xpath and jdk internal classes [v2]

[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.

Joe Wang has updated the pull request incrementally with one additional commit since the last revision:

  replace with instanceof

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/8116/files
  - new: https://git.openjdk.java.net/jdk/pull/8116/files/84a9dceb..5d6a6e3a

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=8116&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=8116&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 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