RFR: 8371964: C2 compilation asserts with "Unexpected load/store size" [v3]

[Quan Anh Mai]

> Hi,
> 
> This fixes the crash in `Load/StoreVectorMaskedNode::Ideal`. The issue here is that the graph is not canonical during idealization, which leads to us processing a dead node. The fix I propose is to bail-out when that happens.
> 
> To be more specific, for this issue, we have the graph that looks like:
> 
>     ConI -> ConvI2L -> CastLL(0..32) -> VectorMaskGen
> 
> with `ConI` being 45 and `MaxVectorSize` being 32. In this instance, `CastLL` is processed before `ConvI2L`, and when it is processed, it sees that the type of `ConvI2L` being its bottom type. As a result, it does not know that it is top, and since we are after macro expansion, which is after loop opts, the `CastLL` goes away, leaving us with:
> 
>     ConI -> ConvI2L -> VectorMaskGen
> 
> After `ConvI2L` is processed, we know that the input of `VectorMaskGen` is a constant 45, which is larger than `MaxVectorSize`, leading to the assert failure.
> 
> Please take a look and leave your thoughts, thanks a lot.

Quan Anh Mai has updated the pull request incrementally with one additional commit since the last revision:

  reviews

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/28410/files
  - new: https://git.openjdk.org/jdk/pull/28410/files/ecaead7f..ec7298ef

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=28410&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=28410&range=01-02

  Stats: 8 lines in 2 files changed: 3 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/28410.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28410/head:pull/28410

PR: https://git.openjdk.org/jdk/pull/28410