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

[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.

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Commit messages:
 - 8329797: Shenandoah: Default case invoked for: "MaxL" (bad AD file)

Changes: https://git.openjdk.org/jdk/pull/18824/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18824&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8329797
  Stats: 48 lines in 5 files changed: 44 ins; 0 del; 4 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