[llvm] [VPlan] Mark unary ops preservesUniformity (PR #215757)

[Luke Lau via llvm-commits]

https://github.com/lukel97 requested changes to this pull request.

This causes regressions in practice, it prevents reverse vlse.vs from being formed on RISC-V. We need to lift the RISC-V vp.reverse combine to VPlan first

https://github.com/llvm/llvm-project/pull/215757