RFR: 8285558: IGV: scheduling crashes on control-unreachable CFG nodes

[Roberto Castañeda Lozano]

This changeset relaxes IGV's schedule approximation algorithm to handle CFG nodes that are not reachable from the root node via a control path. This is done by 1) removing the assumption that, after `ServerCompilerScheduler::buildBlocks()`, `Node::block` is non-null for CFG nodes; and 2) leaving the assignment of blockless nodes to an artificial "no block" to `InputGraph::ensureNodesInBlocks()`, which is always called after running the schedule approximation algorithm.

Additionally, the changeset marks unreachable CFG nodes with a warning, making it easier to identify ill-formed graphs:

![screenshot of IGV graph where some nodes are marked with a warning](https://user-images.githubusercontent.com/8792647/170678998-bffc293f-90ce-45e7-9aea-7cc5ab184026.png)

#### Testing

- Tested manually on the two graphs reported in the [JBS issue](https://bugs.openjdk.java.net/browse/JDK-8285558).

- Tested automatically that scheduling tens of thousands of graphs (by instrumenting IGV to schedule parsed graphs eagerly and running `java -Xcomp -XX:-TieredCompilation -XX:PrintIdealGraphLevel=4`) does not introduce any exception or assertion failure.

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

Commit messages:
 - Warn about control-unreachable CFG nodes
 - Handle control-unreachable nodes

Changes: https://git.openjdk.java.net/jdk/pull/8916/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8916&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8285558
  Stats: 24 lines in 1 file changed: 10 ins; 14 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8916.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8916/head:pull/8916

PR: https://git.openjdk.java.net/jdk/pull/8916