RFR: 8283091: Support type conversion between different data sizes in SLP [v6]

[Fei Gao]

On Fri, 3 Jun 2022 03:13:02 GMT, Vladimir Kozlov <kvn at openjdk.org> wrote:

> May be we should have assert here to make sure that in all places we call `VectorCastNode::opcode()` for `Conv*` nodes:
> 
> ```
>  default:
>     assert(!VectorNode::is_convert_opcode(sopc), "Convert node %s should be processed by VectorCastNode::opcode()", NodeClassNames[sopc]);
>     return 0; // Unimplemented
> ```

Make sense to me. Thanks!

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PR: https://git.openjdk.java.net/jdk/pull/7806