[jdk21u-dev] RFR: 8355387: [jittester] Disable downcasts by default

[Satyen Subramaniam]

Backporting JDK-8355387: [jittester] Disable downcasts by default. Diabling downcasts in jittester code generation. Ran GHA Sanity Checks and new tests directly. Patch is clean. Backporting for parity with Oracle.

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Commit messages:
 - Backport b41e0b17490b203b19787a0d0742318fc0d03b33

Changes: https://git.openjdk.org/jdk21u-dev/pull/2366/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=2366&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8355387
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/2366.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/2366/head:pull/2366

PR: https://git.openjdk.org/jdk21u-dev/pull/2366