RFR: 8338023: Support two vector selectFrom API [v8]

[Jatin Bhateja]

On Fri, 13 Sep 2024 14:53:18 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

> Can you please **define** somewhere what it means to `prune indexes`? It does not help me much more than the previous "massaging indexes" you had before I asked you to change it.
> 
> > Also: I'm a little worried about the semantics change of the RearrangeNode that you did with the changes in RearrangeNode::Ideal. It looks a little "hacky", especially in conjunction with the vector_indexes_needs_massaging method. Can you give a clear definition of the semantics of RearrangeNode and vector_indexes_needs_massaging, please?
> 
> You have also not responded to this yet. It seems to me that before your proposed change, `RearrangeNode` had a clear and easy semantic, and now you somehow "hack it" to work with your `vector_indexes_needs_pruning`. Can you explain please why this makes sense and add a comment to `RearrangeNode` what its semantics is?

In case target does not directly support two vector selection instruction we lower the IR to its constituents, this is better than  intrinsification failure as it saves costly vector boxing penalties. 

Think in terms of desired compiler IR and not rearrange API semantics,  VectorRearrange IR node always expects a shape conformance b/w vector to be permuted and index vector, since shuffle indices are held in byte array based backing storage hence compiler injects VectorLoadShuffle nodes to upcast the byte vector lanes holding indexes to match the input vector lane. 
Since selectFrom API already passes the indexes through vector hence we can save emitting redundant toShuffle() + toVector() operations in all cases apart from some target specific scenarios e.g. AVX2 targets [do not support direct short vector permute]instruction "VPERMW", hence we need to [massage the index vector](https://github.com/openjdk/jdk/blob/master/src/hotspot/cpu/x86/x86.ad#L8771) to emulate desired permutation using byte permute instruction.

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PR Comment: https://git.openjdk.org/jdk/pull/20508#issuecomment-2349801299