RFR: 8310921: Fix -Wconversion warnings from GenerateOopMap

[Coleen Phillimore]

This is broken out from but tested with the PR from https://github.com/openjdk/jdk/pull/14659

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Commit messages:
 - 8310921: Fix -Wconversion warnings from GenerateOopMap

Changes: https://git.openjdk.org/jdk/pull/14666/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14666&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8310921
  Stats: 9 lines in 2 files changed: 0 ins; 0 del; 9 mod
  Patch: https://git.openjdk.org/jdk/pull/14666.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14666/head:pull/14666

PR: https://git.openjdk.org/jdk/pull/14666