RFR: 8267979: C2: Fix verification code in SubTypeCheckNode::Ideal()

[Vladimir Ivanov]

Make verification code in `SubTypeCheckNode::Ideal()` robust in presense of concurrent class loading.

It repeatedly calls `Compile::static_subtype_check()` (directly and indirectly from `SubTypeCheckNode::sub()` through `Value()`) which can switch from `Compile::SSC_easy_test` to `Compile::SSC_full_test` as a result of concurrent class loading. As a result, it breaks the invariant being asserted.

The fix tries to catch such case by checking that `Value()` result is stable across ``Compile::static_subtype_check()` call. 

The patch is split into 2 commits: extensive refactoring and the actual fix on top of it.

Testing:
- [x] hs-tier1 - hs-tier6

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Commit messages:
 - Fix
 - Cleanups

Changes: https://git.openjdk.java.net/jdk/pull/4271/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4271&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8267979
  Stats: 138 lines in 2 files changed: 54 ins; 46 del; 38 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4271.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4271/head:pull/4271

PR: https://git.openjdk.java.net/jdk/pull/4271