RFR: 8342662: C2: Add new phase for backend-specific lowering [v2]

[Jatin Bhateja]

On Sat, 26 Oct 2024 02:34:44 GMT, Quan Anh Mai <qamai at openjdk.org> wrote:

>>> By allowing lowering to look through VectorReinterpret and break the invariant of Extract nodes that the element types of their inputs and outputs must be the same, we can gvn v1 and v, v2 and v0.
>> 
>> I'd warn against breaking existing IR invariants. As an example, precise type information is important to properly match generic ideal vector nodes.
>
> I believe the matcher only needs the exact type of the node but not its inputs. E.g. it should not be an issue if we `AddVB` a `vector<int,8>` and a `vector<long,2>` into a `vector<byte,16>`.

Generic vector operand resolution cocretizes generic operands based on type agnostic node size, its a post matcher pass,  and its job is to replace generic MachOper operand nodes with cocrete ones (vec[SDXYZ]) which holds precise register mask needed by register allocator.

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PR Review Comment: https://git.openjdk.org/jdk/pull/21599#discussion_r1817960219