RFR: 8342095: Add autovectorizer support for subword vector casts

[Jasmine Karthikeyan]

On Wed, 5 Feb 2025 09:25:07 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

>> Originally I thought that larger type conversions were only available on AVX-512 based on the ad-file, but reading it more carefully now I see that they are indeed supported for AVX! I'll go ahead and generalize the changes further to make casting to larger types supported as well. I think this leaves `char` as the only type not supported, which I can look at in a follow-up RFE.
>
> @jaskarth

@eme64 I took a look last night and it looks like supporting casting to larger types (other than `char`) just involves changing the filtering logic in `matcher_x86.hpp` and adding more unit tests, without modifying any of the core logic. I think it could make sense to keep it in the same patch, what do you think? Either way would work with me.

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PR Review Comment: https://git.openjdk.org/jdk/pull/23413#discussion_r1943166874