RFR: 8311813: C1: Uninitialized PhiResolver::_loop field

[Chad Rakoczy]

[JDK-8311813](https://bugs.openjdk.org/browse/JDK-8311813)

Initialize `PhiResolver::_loop` field to `nullptr`

Additional testing:
- [x] Linux x86_64 fastdebug `tier2`
- [x] Linux x86_64 release `tier2`
- [x] Linux x86_64 fastdebug `gtest:all`
- [x] Linux x86_64 release `gtest:all`
- [x] Linux x86_64 fastdebug `test/hotspot/jtreg/compiler/c1`
- [x] Linux x86_64 release `test/hotspot/jtreg/compiler/c1`

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Commit messages:
 - 8311813: C1: Initialize PhiResolver::_loop field to nullptr

Changes: https://git.openjdk.org/jdk/pull/14819/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14819&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8311813
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/14819.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14819/head:pull/14819

PR: https://git.openjdk.org/jdk/pull/14819