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

[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.

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Commit messages:
 - assert in Load/StoreVectorMaskedNode::Ideal

Changes: https://git.openjdk.org/jdk/pull/28410/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28410&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8371964
  Stats: 11 lines in 1 file changed: 9 ins; 0 del; 2 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