RFR: 8310537: Fix -Wconversion warnings in gcUtil.hpp

[Albert Mingkun Yang]

On Wed, 21 Jun 2023 12:16:54 GMT, Coleen Phillimore <coleenp at openjdk.org> wrote:

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

Marked as reviewed by ayang (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/14585#pullrequestreview-1490607437