RFR: 8283091: Support type conversion between different data sizes in SLP [v3]

[Fei Gao]

On Thu, 2 Jun 2022 14:02:33 GMT, Fei Gao <fgao at openjdk.org> wrote:

> @fg1417 I don't see new update in this PR. Please also show performance numbers with new changes

@vnkozlov I updated the patch to resolve the potential performance regression and also updated the performance numbers in the comment.

 But the patch depends on the fix, [8287517: C2: assert(vlen_in_bytes == 64) failed: 2 by sviswa7 · Pull Request #8961 · openjdk/jdk (github.com)](https://github.com/openjdk/jdk/pull/8961), because when we enable the type conversion and loop induction at the same time, we can vectorize more scenarios like https://github.com/openjdk/jdk/blob/6ff2d89ea11934bb13c8a419e7bad4fd40f76759/test/hotspot/jtreg/compiler/c2/cr6340864/TestDoubleVect.java#L723. When we set `MaxVectorSize=16`, the case here would fail. All jtreg tests passed except that one.

Please help review. Thanks.

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