RFR: 8322807: Eliminate -Wparentheses warnings in gc code

[Kim Barrett]

Please review this trivial change to eliminate a -Wparentheses warning.
This involved simply adding parentheses to make the implicit operator
precedence explicit.

Testing: mach5 tier1

Also ran mach5 tier1 with these changes in conjunction enabling -Wparentheses
and other changes needed to make that work.

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Commit messages:
 - fix -Wparentheses warnings in gc code

Changes: https://git.openjdk.org/jdk/pull/17212/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17212&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8322807
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/17212.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17212/head:pull/17212

PR: https://git.openjdk.org/jdk/pull/17212