RFR: 8263227: C2: inconsistent spilling due to dead nodes in exception block

[Roberto Castañeda Lozano]

This change eliminates dead multi-nodes created by call-catch cleanup after GCM. Eliminating all dead code created by call-catch cleanup avoids potential issues when splitting the live range of call result values, see the analysis in the [bug report](https://bugs.openjdk.java.net/browse/JDK-8263227) for details. This solution is the least invasive of the three alternatives proposed in the bug report (the other two are constraining global code motion and extending live-range splitting).

The change also extends the control-flow graph verification pass to catch similar live-range splitting issues earlier (with `+VerifyRegisterAllocator`).

Tested on:
- original bug reproducer
- hs-tier1-5 (windows-x64, linux-x64, linux-aarch64, and macosx-x64) with `+VerifyRegisterAllocator`
- hs-tier1-3 (windows-x64, linux-x64, linux-aarch64, and macosx-x64) with `+VerifyRegisterAllocator` and `+StressGCM` (5 repetitions)

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Commit messages:
 - Update comments in test case
 - Exclude MachMerge nodes from dominance assertion
 - Remove cloned multi-nodes in call-catch cleanup
 - Document assumptions in live-range splitting
 - Add basic definition dominance assertion
 - Add test case

Changes: https://git.openjdk.java.net/jdk/pull/3303/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3303&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8263227
  Stats: 102 lines in 4 files changed: 95 ins; 0 del; 7 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3303.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3303/head:pull/3303

PR: https://git.openjdk.java.net/jdk/pull/3303