[jdk21u] RFR: 8311813: C1: Uninitialized PhiResolver::_loop field

[Chad Rakoczy]

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

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

Clean backport

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Commit messages:
 - Backport 489a32fe40e2a2c539296d51d4ffc0abc036d33c

Changes: https://git.openjdk.org/jdk21u/pull/141/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk21u&pr=141&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/jdk21u/pull/141.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u.git pull/141/head:pull/141

PR: https://git.openjdk.org/jdk21u/pull/141