RFR: 8310921: Fix -Wconversion warnings from GenerateOopMap [v2]

[Coleen Phillimore]

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

Coleen Phillimore has updated the pull request incrementally with one additional commit since the last revision:

  David Holmes comments

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Changes:
  - all: https://git.openjdk.org/jdk/pull/14666/files
  - new: https://git.openjdk.org/jdk/pull/14666/files/3902331a..84d64a6b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=14666&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=14666&range=00-01

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 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