RFR: 8306456: Don't leak _worklist's memory in PhaseLive::compute

[Johan Sjölen]

`PhaseLive::compute` used to do this: `_worklist = new (_arena) Block_List();`. This allocates the `Block_List` to the `_arena`, but the backing array is allocated on the resource area: `Block_List() : Block_Array(Thread::current()->resource_area()), _cnt(0) {}`. This causes at most 5 worklists and at least 4 worklists to be created and not freed until the compilation is finished. This patch allocates the worklist within `PhaseLive::compute`:s local resource mark.

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Commit messages:
 - Don't leak the worklist in PhaseLive

Changes: https://git.openjdk.org/jdk/pull/13535/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13535&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8306456
  Stats: 5 lines in 1 file changed: 3 ins; 1 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/13535.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13535/head:pull/13535

PR: https://git.openjdk.org/jdk/pull/13535