RFR: 8327201: C2: Uninitialized VLoop::_pre_loop_end after JDK-8324890

[Joshua Cao]

As Aleksey pointed out, the issue seems innocuous. It seems that all code that uses `pre_loop_end` are called from the main loop, and the field is always initialized for main loops. But we should still avoid uninitialized fields.

Passing hotspot tier1 locally on my Linux machine.

-------------

Commit messages:
 - 8327201: C2: Uninitialized VLoop::_pre_loop_end after JDK-8324890

Changes: https://git.openjdk.org/jdk/pull/18130/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18130&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8327201
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/18130.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18130/head:pull/18130

PR: https://git.openjdk.org/jdk/pull/18130