[llvm] [SLP][REVEC] Enable vectorisation of vector_extract intrinsics (PR #148820)

[Gaëtan Bossu via llvm-commits]

gbossu wrote:

Okay I had missed the big change introduced recently in #148007. Now the SLPVectorizer doesn't generate `vector.extract` intrinsics anymore, so this makes this patch a lot less useful... I rebased to update the tests, but I'll check if this patch needs updating in a different way now.

https://github.com/llvm/llvm-project/pull/148820