RFR: 8299179: ArrayFill with store on backedge needs to reduce length by 1 [v2]

[Emanuel Peter]

> **Context**
> `ArrayFill` replaces `CountedLoop` like this:
> `for(int i = 5; i < 15; i++) { arr[i] = 88; }`
> with
> `jint_fill(arr, 5, len, 88)` (CallLeafNoFPNode, intrinsified)
> where `len = limit - init`.
> This is implemented for `byte`, `short` and `int` (`long` not yet supported).
> 
> **Solution**
> This assumes that the `store` is in the body of the loop. In rare cases, the `store` is pinned to the backedge, and so it is executed after the exit check, hence in all iterations except the last one. In those cases we need to set `len = limit - init - 1`.
> 
> **Details**
> _How does a store land on the backedge?_
> `test_002` was first reported to me. I reduced it down to a conceptually easier `test_102`.
> We have a loop, where the first block only has a single successor. This block contains the `store` that it eventually detected as a `ArrayFill`. This block passes the checks of `clone_loop_head` during parsing: the block is copied before the loop and onto the backedge.
> 
> **FYI**
> I saw that we seem to currently mostly unroll such loops, instead of applying the `ArrayFill`. Follow up RFE [JDK-8299808](https://bugs.openjdk.org/browse/JDK-8299808).

Emanuel Peter has updated the pull request incrementally with one additional commit since the last revision:

  Fix indentation
  
  Co-authored-by: Tobias Hartmann <tobias.hartmann at oracle.com>

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/11904/files
  - new: https://git.openjdk.org/jdk/pull/11904/files/6cd85641..a9259236

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=11904&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=11904&range=00-01

  Stats: 4 lines in 1 file changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/11904.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11904/head:pull/11904

PR: https://git.openjdk.org/jdk/pull/11904