RFR: 8303279: C2: crash in SubTypeCheckNode::sub() at IGVN split if [v2]

[Roland Westrelin]

> The crash occurs because at split if during IGVN, a `SubTypeCheck` is
> created with null as input. That happens because the control path the
> `SubTypeCheck` is cloned for is dead. To fix that I propose delaying
> split if until dead paths are collapsed.
> 
> I added an assert to check a nullable first input to `SubTypeCheck`
> nodes (which should be impossible because it should be null
> checked). When I ran testing, a number of cases showed up with known
> non null values non properly marked as non null. I fixed them.

Roland Westrelin has updated the pull request incrementally with two additional commits since the last revision:

 - review
 - review

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/14678/files
  - new: https://git.openjdk.org/jdk/pull/14678/files/cb3a04f0..0cf15596

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=14678&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=14678&range=00-01

  Stats: 7 lines in 3 files changed: 4 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/14678.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14678/head:pull/14678

PR: https://git.openjdk.org/jdk/pull/14678