RFR: 8251271: C2: Compile::_for_igvn list is corrupted after RenumberLiveNodes

[Nhat Nguyen]

I'm following up on this thread http://mail.openjdk.java.net/pipermail/hotspot-compiler-dev/2020-September/039910.html
I'm clearing `for_igvn` before restoring as suggested by Xin

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Commit messages:
 - 8251271: C2: Compile::_for_igvn list is corrupted after RenumberLiveNodes

Changes: https://git.openjdk.java.net/jdk/pull/713/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=713&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8251271
  Stats: 7 lines in 1 file changed: 7 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/713.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/713/head:pull/713

PR: https://git.openjdk.java.net/jdk/pull/713