[clang] [llvm] Disable pre-link LTO vectorisation and loop unroll (PR #195886)

[Luke Lau via cfe-commits]

lukel97 wrote:

I checked with all the patches in this series combined and I'm still seeing a 12% regression with 538.imagick_r on RISC-V. The regression comes from no longer SLP vectorizing the hottest block, since LoopUnroll now runs before it.

I'm not sure why unrolling the loop prevents SLP from vectorizing it. We could explore moving LoopUnroll after SLP. Or alternatively look into getting SLP to kick in for the unrolled form.

https://github.com/llvm/llvm-project/pull/195886