RFR: 8337031: Improvements to CompilationMemoryStatistic

[Ashutosh Mehra]

Some minor improvements to CompilationMemoryStatistic. More details are in [JDK-8337031](https://bugs.openjdk.org/browse/JDK-8337031)

Testing:
  test/hotspot/jtreg/compiler/print/CompileCommandPrintMemStat.java
  test/hotspot/jtreg/serviceability/dcmd/compiler/CompilerMemoryStatisticTest.java

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Commit messages:
 - Update comments in tests to reflect new output format
 - 8337031: Improvements to CompilationMemoryStatistic

Changes: https://git.openjdk.org/jdk/pull/20304/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20304&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8337031
  Stats: 173 lines in 6 files changed: 77 ins; 21 del; 75 mod
  Patch: https://git.openjdk.org/jdk/pull/20304.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20304/head:pull/20304

PR: https://git.openjdk.org/jdk/pull/20304