RFR: 7903132: Replace casts with type test pattern

[Ethan McCue]

Replaces a handful of casts following type tests with the equivalent pattern syntax.

I cannot create a matching issue, as I do not have an account on the bug tracker.

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Commit messages:
 - Replace casts with type test pattern

Changes: https://git.openjdk.java.net/jextract/pull/5/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jextract&pr=5&range=00
  Issue: https://bugs.openjdk.java.net/browse/CODETOOLS-7903132
  Stats: 81 lines in 13 files changed: 2 ins; 22 del; 57 mod
  Patch: https://git.openjdk.java.net/jextract/pull/5.diff
  Fetch: git fetch https://git.openjdk.java.net/jextract pull/5/head:pull/5

PR: https://git.openjdk.java.net/jextract/pull/5