[llvm] [RISCV][TTI] Reduce cost of a build_vector pattern (PR #108419)

[Philip Reames via llvm-commits]

preames wrote:

> One example I found was in `pov::compute_backtrace_texture(float*, pov::Texture_Struct*, double*, double*, pov::Ray_Struct*, double, pov::istk_entry*) (_ZN3povL25compute_backtrace_textureEPfPNS_14Texture_StructEPdS3_PNS_10Ray_StructEdPNS_10istk_entryE)`

I spent some time looking at this routine yesterday, and am still digging today.  I've been focusing less on what the current codegen looks like, than what optimal code for this sequence might look like.  (Mostly because I was looking at a differently compiled binary.)  I see a couple of possible improvements which would help both before and after significantly, but none of them are easy.  I'm going to switch and look more closely at the regression today.

https://github.com/llvm/llvm-project/pull/108419