8387073: C2: StoreNode::Ideal wrongly optimizes away scalar store before masked vector store

[mhaessig]

StoreNode::Ideal() optimizes back-to-back stores to the same address by removing the earlier store if it is the same size or narrower than the later store when we have UseAVX >= 3 or UseSVE > 0. This optimization is incorrect if the later store is a masked vector store as it might not overwrite all bits of the preceding store.

This PR fixes this issue by plainly disabling the optimization for masked stores. We could conceivably enable the optimization for masked stores if we can prove that the mask overwrites all of the preceding store, but this fix is aimed at correctness before the 27 release. Thus, such optimizations are filed as follow-up RFEs.

Testing:

• Github Actions
• tier1,tier2,tier3 plus additional stress testing and an additional run on AVX-512 machines only.
• tier4,tier5

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Issue

• JDK-8387073: C2: StoreNode::Ideal wrongly optimizes away scalar store before masked vector store (Bug - P4)(⚠️ The fixVersion in this issue is [27] but the fixVersion in .jcheck/conf is 28, a new backport will be created when this pr is integrated.)

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