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

[Philip Reames via llvm-commits]

preames wrote:

I figured out the root cause of the Leela thing, and filed a ticket so we don't forget.  This is much sillier than we'd first realized.  Vectorizing this is a quite terrible result, but seems to have little to do with this patch.  https://github.com/llvm/llvm-project/issues/109466

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