[llvm] [RISCV][TTI] Use legalized element types when costing casts (PR #105723)

[Philip Reames via llvm-commits]

preames wrote:

> Is this the fix for #104048

Very likely.  I hadn't started from that, but the failure symptom looks very similar to my crash.

https://github.com/llvm/llvm-project/pull/105723