RFR: JDK-8330565 : C2: Multiple crashes with CTW after JDK-8316991 [v3]

[Cesar Soares Lucas]

> The `# assert(false) failed: Bad graph detected in build_loop_late` failure was caused because a string concatenation optimization using [this method](https://github.com/openjdk/jdk/blob/819f3d6fc70ff6fe54ac5f9033c17c3dd4326aa5/src/hotspot/share/opto/graphKit.cpp#L4115) adds AddP and LoadN nodes to IR graph as NotNull _and_ because RAM was not "nullyfing" phis merging nullable pointers. I was only able to reproduce this problem using a classfile/jar compiled using an "old" version of JDK.. because newer version use InvokeDynamic to do string concatenation.
> 
> Tested with JTREG tier1-4 on Linux x86_64 & ARM64.

Cesar Soares Lucas has updated the pull request incrementally with one additional commit since the last revision:

  Update test/hotspot/jtreg/compiler/c2/TestReduceAllocationAndNullableLoads.java
  
  -Xcomp implies -Xbatch
  
  Co-authored-by: Tobias Hartmann <tobias.hartmann at oracle.com>

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/19147/files
  - new: https://git.openjdk.org/jdk/pull/19147/files/94eb0e12..facc93b9

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=19147&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=19147&range=01-02

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/19147.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19147/head:pull/19147

PR: https://git.openjdk.org/jdk/pull/19147