RFR: 8344798: Shenandoah: Use more descriptive variable names in shPhaseTimings.cpp

[William Kemper]

The single letter variable names make some of this code harder to read.

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Commit messages:
 - Use more descriptive variable names

Changes: https://git.openjdk.org/jdk/pull/22310/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22310&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8344798
  Stats: 14 lines in 1 file changed: 0 ins; 0 del; 14 mod
  Patch: https://git.openjdk.org/jdk/pull/22310.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22310/head:pull/22310

PR: https://git.openjdk.org/jdk/pull/22310