RFR: 8312596: Null pointer access in Compile::TracePhase::~TracePhase after JDK-8311976 [v4]

[Ashutosh Mehra]

> Please review this PR to fix a potential null pointer access in using `_compile`.
> Updated the code to unconditionally initialize `_compile` and added an assert (similar to C1's `PhaseTraceTime` constructor) for it to be non-null.

Ashutosh Mehra has updated the pull request incrementally with one additional commit since the last revision:

  Update test config to use vm.compiler2.enabled
  
  Signed-off-by: Ashutosh Mehra <asmehra at redhat.com>

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Changes:
  - all: https://git.openjdk.org/jdk/pull/15002/files
  - new: https://git.openjdk.org/jdk/pull/15002/files/70f3ebad..5e878d97

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=15002&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=15002&range=02-03

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/15002.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15002/head:pull/15002

PR: https://git.openjdk.org/jdk/pull/15002