RFR: 8308583: SIGSEGV in GraphKit::gen_checkcast [v2]

[Roland Westrelin]

> At an `instanceof`, a node of type `bottom[int:>=0]` is checked to be of type `cc$Word` and on the success a `CheckCastPP` is inserted to change the node type. That `CheckCastPP` constant folds to `top` but the type check doesn't fold. The reason is that the type check loads the klass from the node with a `LoadNKlass` and the type of that node is `java/lang/Object`  when it should be `bottom[int:>=0]` but logic in `LoadNKlass::Value()` gets in the way.

Roland Westrelin has updated the pull request incrementally with one additional commit since the last revision:

  review

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/14123/files
  - new: https://git.openjdk.org/jdk/pull/14123/files/a886d516..75de9a72

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=14123&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=14123&range=00-01

  Stats: 8 lines in 1 file changed: 1 ins; 0 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/14123.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14123/head:pull/14123

PR: https://git.openjdk.org/jdk/pull/14123