[llvm] [SLP][REVEC] Make SLP support revectorization (-slp-revec) and add simple test. (PR #98269)

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dyung wrote:

Hi @HanKuanChen, we have several internal tests that are hitting an assertion failure which I bisected back to this change. I have filed PR #99411 for the issue, can you take a look?

https://github.com/llvm/llvm-project/pull/98269