RFR: 8310537: Fix -Wconversion warnings in gcUtil.hpp [v3]

[Coleen Phillimore]

> Please review small changes to fix -Wconversion warnings in gc code.  See CR for details.
> Tested with tier1 on Oracle platforms.

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

  Add a comment about the float conversion.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/14585/files
  - new: https://git.openjdk.org/jdk/pull/14585/files/f4133e65..01d75316

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=14585&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=14585&range=01-02

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/14585.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14585/head:pull/14585

PR: https://git.openjdk.org/jdk/pull/14585