[jdk16u] RFR: 8261912: Code IfNode::fold_compares_helper more defensively

[Aleksey Shipilev]

This fixes C2 miscompilation. Risk is low, as it limits the optimization where it would have miscompiled.

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Commit messages:
 - Backport e9f3aab7f483818944faf8638f421c6960fafbd2

Changes: https://git.openjdk.java.net/jdk16u/pull/48/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk16u&pr=48&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261912
  Stats: 23 lines in 1 file changed: 15 ins; 4 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk16u/pull/48.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk16u pull/48/head:pull/48

PR: https://git.openjdk.java.net/jdk16u/pull/48