[llvm] [SLP] Reland: More accurately cost RISCV scalar splats (PR #224766)

[Luke Lau via llvm-commits]

lukel97 wrote:

> > After this lands would you mind keeping an eye on LNT tomorrow and seeing if the performance is affected?
> 
> Looks like there are some regressions, I'll look into them: https://cc-perf.igalia.com/db_default/v4/nts/1117

Thanks, it seems like the povray regression was recoverered but namd and x264 are still regressed from the looks of things: https://cc-perf.igalia.com/db_default/v4/nts/1123?compare_to=1105

https://github.com/llvm/llvm-project/pull/224766