[jdk16] RFR: 8253353: Crash in C2: guarantee(n != NULL) failed: No Node

[Vladimir Kozlov]

Nesting loops for this case reaches 400+. But the type of IdeaLoopTree::_nest field is uint8 (the maximum value allowed is 255). The test passed with the following fix proposed by @RealFYang:

- uint8_t _nest; // Nesting depth 
+ uint16_t _nest; // Nesting depth

New regression test is added.
Passed hs-tier1 - 3 testing.

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Commit messages:
 - 8253353: Crash in C2: guarantee(n != NULL) failed: No Node

Changes: https://git.openjdk.java.net/jdk16/pull/135/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk16&pr=135&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8253353
  Stats: 1987 lines in 4 files changed: 1986 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk16/pull/135.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk16 pull/135/head:pull/135

PR: https://git.openjdk.java.net/jdk16/pull/135