RFR: 8329797: Shenandoah: Default case invoked for: "MaxL" (bad AD file) [v3]

[Joshua Cao]

> The bug occurs when [Shenandoah optimizations resets post_loop_opts](https://github.com/openjdk/jdk/blob/040c93565c0dff6270911eb9e58d78aa01bbb925/src/hotspot/share/gc/shenandoah/c2/shenandoahSupport.cpp#L52), and we may [create a MaxL after macro expansion](https://github.com/openjdk/jdk/blob/040c93565c0dff6270911eb9e58d78aa01bbb925/src/hotspot/share/opto/movenode.cpp#L198). `MaxL` does not have a matcher rule, and we run into an assertion failure.
> 
> This PR guards the `MaxL` creation with a new `began_macro_expansion()` flag. I think there are many other instances in code that should use the new flag instead of `post_loop_opts()`, which can be explored in [JDK-8330531](https://bugs.openjdk.org/browse/JDK-8330531).
> 
> The bug was originally found in [h2 Index::getCostRangeIndex()](https://github.com/h2database/h2database/blob/master/h2/src/main/org/h2/index/Index.java#L579) through Dacapo. Its easy to reproduce by creating a loop that includes a `ShenandoahLoadReferenceBarrier` (load any object) and a `MaxL`.
> 
> Caveat: I created test cases for both `MaxL` and `MinL` for completeness. The `MinL` test case does not actually fail before this PR. Somehow the `CMove` condition is converted to non-canonical `>`, which is [not accepted by the Idealization](https://github.com/openjdk/jdk/blob/040c93565c0dff6270911eb9e58d78aa01bbb925/src/hotspot/share/opto/movenode.cpp#L219). The `MinL` is never created and there is no crash.
> 
> Passing hotspot tier1 locally on Linux machine.

Joshua Cao has updated the pull request incrementally with one additional commit since the last revision:

  Update comment for MinL/MaxL based on renaming of allow_macro_nodes

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/18824/files
  - new: https://git.openjdk.org/jdk/pull/18824/files/4cacffe9..88c3c1d9

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18824&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18824&range=01-02

  Stats: 2 lines in 1 file changed: 0 ins; 1 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/18824.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18824/head:pull/18824

PR: https://git.openjdk.org/jdk/pull/18824